cmake: encapsulate enabling/disabling compiler warnings

There are multiple sites where we enable or disable compiler warning via
"-W<warning>" or "-Wno-<warning>". As we want to extend this mechanism
later on to conditionally switch these over to "-Werror=<warning>", we
encapsulate the logic into its their own macros `ENABLE_WARNINGS` and
`DISABLE_WARNINGS`.

Note that we in fact have to use a macro here. Using a function would
not modify the CFLAGS inside of the callers scope, but in the function's
scope only.

CMakeLists.txt

@@ -146,6 +146,14 @@ ELSE(IS_ABSOLUTE ${INCLUDE_INSTALL_DIR})
   SET (PKGCONFIG_INCLUDEDIR "\${prefix}/${INCLUDE_INSTALL_DIR}")
 ENDIF(IS_ABSOLUTE ${INCLUDE_INSTALL_DIR})
 
+MACRO(ENABLE_WARNINGS flag)
+	ADD_C_FLAG_IF_SUPPORTED(-W${flag})
+ENDMACRO()
+
+MACRO(DISABLE_WARNINGS flag)
+	ADD_C_FLAG_IF_SUPPORTED(-Wno-${flag})
+ENDMACRO()
+
 FUNCTION(TARGET_OS_LIBRARIES target)
 	IF(WIN32)
 		TARGET_LINK_LIBRARIES(${target} ws2_32)
@@ -471,8 +479,8 @@ IF (MSVC)
 ELSE ()
 	SET(CMAKE_C_FLAGS "-D_GNU_SOURCE ${CMAKE_C_FLAGS}")
 
-	ADD_C_FLAG_IF_SUPPORTED(-Wall)
-	ADD_C_FLAG_IF_SUPPORTED(-Wextra)
+	ENABLE_WARNINGS(all)
+	ENABLE_WARNINGS(extra)
 
 	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}")
@@ -495,16 +503,16 @@ ELSE ()
 		ADD_DEFINITIONS(-D__USE_MINGW_ANSI_STDIO=1)
 	ENDIF ()
 
-	ADD_C_FLAG_IF_SUPPORTED(-Wdocumentation)
-	ADD_C_FLAG_IF_SUPPORTED(-Wno-missing-field-initializers)
-	ADD_C_FLAG_IF_SUPPORTED(-Wstrict-aliasing=2)
-	ADD_C_FLAG_IF_SUPPORTED(-Wstrict-prototypes)
-	ADD_C_FLAG_IF_SUPPORTED(-Wdeclaration-after-statement)
-	ADD_C_FLAG_IF_SUPPORTED(-Wno-unused-const-variable)
-	ADD_C_FLAG_IF_SUPPORTED(-Wno-unused-function)
+	ENABLE_WARNINGS(documentation)
+	ENABLE_WARNINGS(strict-aliasing=2)
+	ENABLE_WARNINGS(strict-prototypes)
+	ENABLE_WARNINGS(declaration-after-statement)
+	DISABLE_WARNINGS(missing-field-initializers)
+	DISABLE_WARNINGS(unused-const-variable)
+	DISABLE_WARNINGS(unused-function)
 
 	IF (APPLE) # Apple deprecated OpenSSL
-		ADD_C_FLAG_IF_SUPPORTED(-Wno-deprecated-declarations)
+		DISABLE_WARNINGS(deprecated-declarations)
 	ENDIF()
 
 	IF (PROFILE)