cmake: use pkg_check_modules() for GnuTLS

Remove the local cmake file and associated hacks used to manage GnuTLS.
This gives us less build-system glue code and makes it easier to enforce
a minimum version for GnuTLS.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

CMakeLists.txt

@@ -211,11 +211,9 @@ include_directories(${LIBGCRYPT_INCLUDE_DIRS})
 list(APPEND EXTRA_LIBS ${LIBGCRYPT_LDFLAGS})
 
 # Check for GnuTLS
-find_package(GnuTLS REQUIRED)
-string(REGEX REPLACE "/[^/]*$" "" GNUTLS_LIBRARY_PATH "${GNUTLS_LIBRARY}")
-include_directories(${GNUTLS_INCLUDE_PATH})
-set(CMAKE_C_LINK_FLAGS "${CMAKE_C_LINK_FLAGS} -L${GNUTLS_LIBRARY_PATH}")
-list(APPEND EXTRA_LIBS gnutls)
+pkg_check_modules(GNUTLS REQUIRED gnutls)
+include_directories(${GNUTLS_INCLUDE_DIRS})
+list(APPEND EXTRA_LIBS ${GNUTLS_LDFLAGS})
 
 # Check for zlib
 find_package(ZLIB REQUIRED)