When using clang, warn about c99 only syntax (like pedantic without all the rest).

Change-Id: Ife6fcb14ce04e4d505ee4911a102420932fb18de
Reviewed-on: https://code.wireshark.org/review/15334
Reviewed-by: Jörg Mayer <jmayer@loplof.de>

CMakeLists.txt

@@ -382,6 +382,8 @@ else()
 		# The Qt headers generate a ton of shortening errors on 64-bit systems
 		# so only enable this for C for now.
 		-Wshorten-64-to-32
+		# Clang only
+		-Wc99-extensions
 	)
 
 	set(WIRESHARK_CXX_ONLY_FLAGS