Hardening: add -fstrict-flex-arrays=3

This should help gcc/clang with finding more OOB write bugs.

It does mean that 3rd party modules can no longer use the something like:
struct { char name[1]; }
and then alloc(sizeof(struct) + length of name)

instead the struct element needs to be name[];
...and they would need to alloc(sizeof(struct) + length of name + 1)

No 3rd party modules in unrealircd-contrib use this so.. hopefully fine.

configure.ac

@@ -147,6 +147,9 @@ AS_IF([test x"$hardening" != x"no"], [
   # Control Flow Enforcement (ROP hardening) - requires CPU hardware support
   check_cc_flag([-fcf-protection], [HARDEN_CFLAGS="$HARDEN_CFLAGS -fcf-protection"])
 
+  # Since we have moved from name[1] to name[], setting this should help bounds checking:
+  check_cc_flag([-fstrict-flex-arrays=3], [HARDEN_CFLAGS="$HARDEN_CFLAGS -fstrict-flex-arrays=3"])
+
   # At the link step, we might want -pie (GCC) or -Wl,-pie (Clang on OS X)
   #
   # The linker checks also compile code, so we need to include -fPIE as well.