cmake: only enable supported compiler warning flags

We currently unconditionally enable the "-Wall" and "-Wextra" flags.
Some platforms rely on compilers which do not support these flags,
though. One of these platforms is Haiku, which does not support
"-Wextra" due to being stuck on GCC version 2.

Fix builds on such platforms by adding these flags only if supported by
the compiler.

CMakeLists.txt

@@ -464,7 +464,10 @@ IF (MSVC)
    # Precompiled headers
 
 ELSE ()
-	SET(CMAKE_C_FLAGS "-D_GNU_SOURCE -Wall -Wextra ${CMAKE_C_FLAGS}")
+	SET(CMAKE_C_FLAGS "-D_GNU_SOURCE ${CMAKE_C_FLAGS}")
+
+	ADD_C_FLAG_IF_SUPPORTED(-Wall)
+	ADD_C_FLAG_IF_SUPPORTED(-Wextra)
 
 	IF (CMAKE_SYSTEM_NAME MATCHES "(Solaris|SunOS)")
 		SET(CMAKE_C_FLAGS "-std=c99 -D_POSIX_C_SOURCE=200112L -D__EXTENSIONS__ -D_POSIX_PTHREAD_SEMANTICS ${CMAKE_C_FLAGS}")