diff --git a/configure b/configure
index 71d9ae2c6..96b505c05 100755
--- a/configure
+++ b/configure
@@ -3499,6 +3499,187 @@ ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
 ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
+   { $as_echo "$as_me:${as_lineno-$LINENO}: checking for $CC option to accept ISO C99" >&5
+$as_echo_n "checking for $CC option to accept ISO C99... " >&6; }
+if ${ac_cv_prog_cc_c99+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_cv_prog_cc_c99=no
+ac_save_CC=$CC
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <wchar.h>
+#include <stdio.h>
+
+// Check varargs macros.  These examples are taken from C99 6.10.3.5.
+#define debug(...) fprintf (stderr, __VA_ARGS__)
+#define showlist(...) puts (#__VA_ARGS__)
+#define report(test,...) ((test) ? puts (#test) : printf (__VA_ARGS__))
+static void
+test_varargs_macros (void)
+{
+  int x = 1234;
+  int y = 5678;
+  debug ("Flag");
+  debug ("X = %d\n", x);
+  showlist (The first, second, and third items.);
+  report (x>y, "x is %d but y is %d", x, y);
+}
+
+// Check long long types.
+#define BIG64 18446744073709551615ull
+#define BIG32 4294967295ul
+#define BIG_OK (BIG64 / BIG32 == 4294967297ull && BIG64 % BIG32 == 0)
+#if !BIG_OK
+  your preprocessor is broken;
+#endif
+#if BIG_OK
+#else
+  your preprocessor is broken;
+#endif
+static long long int bignum = -9223372036854775807LL;
+static unsigned long long int ubignum = BIG64;
+
+struct incomplete_array
+{
+  int datasize;
+  double data[];
+};
+
+struct named_init {
+  int number;
+  const wchar_t *name;
+  double average;
+};
+
+typedef const char *ccp;
+
+static inline int
+test_restrict (ccp restrict text)
+{
+  // See if C++-style comments work.
+  // Iterate through items via the restricted pointer.
+  // Also check for declarations in for loops.
+  for (unsigned int i = 0; *(text+i) != '\0'; ++i)
+    continue;
+  return 0;
+}
+
+// Check varargs and va_copy.
+static void
+test_varargs (const char *format, ...)
+{
+  va_list args;
+  va_start (args, format);
+  va_list args_copy;
+  va_copy (args_copy, args);
+
+  const char *str;
+  int number;
+  float fnumber;
+
+  while (*format)
+    {
+      switch (*format++)
+	{
+	case 's': // string
+	  str = va_arg (args_copy, const char *);
+	  break;
+	case 'd': // int
+	  number = va_arg (args_copy, int);
+	  break;
+	case 'f': // float
+	  fnumber = va_arg (args_copy, double);
+	  break;
+	default:
+	  break;
+	}
+    }
+  va_end (args_copy);
+  va_end (args);
+}
+
+int
+main ()
+{
+
+  // Check bool.
+  _Bool success = false;
+
+  // Check restrict.
+  if (test_restrict ("String literal") == 0)
+    success = true;
+  char *restrict newvar = "Another string";
+
+  // Check varargs.
+  test_varargs ("s, d' f .", "string", 65, 34.234);
+  test_varargs_macros ();
+
+  // Check flexible array members.
+  struct incomplete_array *ia =
+    malloc (sizeof (struct incomplete_array) + (sizeof (double) * 10));
+  ia->datasize = 10;
+  for (int i = 0; i < ia->datasize; ++i)
+    ia->data[i] = i * 1.234;
+
+  // Check named initializers.
+  struct named_init ni = {
+    .number = 34,
+    .name = L"Test wide string",
+    .average = 543.34343,
+  };
+
+  ni.number = 58;
+
+  int dynamic_array[ni.number];
+  dynamic_array[ni.number - 1] = 543;
+
+  // work around unused variable warnings
+  return (!success || bignum == 0LL || ubignum == 0uLL || newvar[0] == 'x'
+	  || dynamic_array[ni.number - 1] != 543);
+
+  ;
+  return 0;
+}
+_ACEOF
+for ac_arg in '' -std=gnu99 -std=c99 -c99 -AC99 -D_STDC_C99= -qlanglvl=extc99
+do
+  CC="$ac_save_CC $ac_arg"
+  if ac_fn_c_try_compile "$LINENO"; then :
+  ac_cv_prog_cc_c99=$ac_arg
+fi
+rm -f core conftest.err conftest.$ac_objext
+  test "x$ac_cv_prog_cc_c99" != "xno" && break
+done
+rm -f conftest.$ac_ext
+CC=$ac_save_CC
+
+fi
+# AC_CACHE_VAL
+case "x$ac_cv_prog_cc_c99" in
+  x)
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: none needed" >&5
+$as_echo "none needed" >&6; } ;;
+  xno)
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: unsupported" >&5
+$as_echo "unsupported" >&6; } ;;
+  *)
+    CC="$CC $ac_cv_prog_cc_c99"
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_prog_cc_c99" >&5
+$as_echo "$ac_cv_prog_cc_c99" >&6; } ;;
+esac
+if test "x$ac_cv_prog_cc_c99" != xno; then :
+
+fi
+
+
+if test "$ac_cv_prog_cc_c99" = "no"; then :
+  as_fn_error $? "No C99 compiler was found. Please install gcc or clang and probably more. Eg, on Debian/Ubuntu you probably want to run: apt-get install build-essential " "$LINENO" 5
+fi
+
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for crypt in -ldescrypt" >&5
 $as_echo_n "checking for crypt in -ldescrypt... " >&6; }
@@ -5401,48 +5582,6 @@ fi
 done
 
 
-if test $ac_cv_c_compiler_gnu = yes; then
-    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC needs -traditional" >&5
-$as_echo_n "checking whether $CC needs -traditional... " >&6; }
-if ${ac_cv_prog_gcc_traditional+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-    ac_pattern="Autoconf.*'x'"
-  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
-/* end confdefs.h.  */
-#include <sgtty.h>
-Autoconf TIOCGETP
-_ACEOF
-if (eval "$ac_cpp conftest.$ac_ext") 2>&5 |
-  $EGREP "$ac_pattern" >/dev/null 2>&1; then :
-  ac_cv_prog_gcc_traditional=yes
-else
-  ac_cv_prog_gcc_traditional=no
-fi
-rm -f conftest*
-
-
-  if test $ac_cv_prog_gcc_traditional = no; then
-    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
-/* end confdefs.h.  */
-#include <termio.h>
-Autoconf TCGETA
-_ACEOF
-if (eval "$ac_cpp conftest.$ac_ext") 2>&5 |
-  $EGREP "$ac_pattern" >/dev/null 2>&1; then :
-  ac_cv_prog_gcc_traditional=yes
-fi
-rm -f conftest*
-
-  fi
-fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_prog_gcc_traditional" >&5
-$as_echo "$ac_cv_prog_gcc_traditional" >&6; }
-  if test $ac_cv_prog_gcc_traditional = yes; then
-    CC="$CC -traditional"
-  fi
-fi
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether setpgrp takes no argument" >&5
 $as_echo_n "checking whether setpgrp takes no argument... " >&6; }
 if ${ac_cv_func_setpgrp_void+:} false; then :
diff --git a/configure.ac b/configure.ac
index 7aa2c97d1..f89d1d817 100644
--- a/configure.ac
+++ b/configure.ac
@@ -66,6 +66,11 @@ AC_PATH_PROG(GMAKE,gmake)
 AC_PATH_PROG(GUNZIP, gunzip)
 AC_PATH_PROG(PKGCONFIG, pkg-config)
 
+dnl Check for compiler
+AC_PROG_CC_C99
+AS_IF([test "$ac_cv_prog_cc_c99" = "no"],
+	[AC_MSG_ERROR([No C99 compiler was found. Please install gcc or clang and other build tools. Eg, on Debian/Ubuntu you probably want to run the following as root: apt-get install build-essential ])])
+
 dnl Checks for libraries.
 AC_CHECK_LIB(descrypt, crypt,
 	[AC_DEFINE([HAVE_CRYPT], [], [Define if you have crypt])
@@ -317,7 +322,6 @@ AC_CHECK_HEADER(glob.h,
 AC_CHECK_HEADERS([stdint.h inttypes.h])
 
 dnl Checks for library functions.
-AC_PROG_GCC_TRADITIONAL
 AC_FUNC_SETPGRP
 AC_FUNC_SETVBUF_REVERSED
 AC_CHECK_FUNCS(strlcpy,