cmake: add switch to build with -Werror

Add a simple switch to enable building with "-Werror=<warning>" instead
of "-W<warning". Due to the encapsulated `ENABLE_WARNINGS` macro, this
is as simple as adding a new variable "ENABLE_WERROR`, which can be
passed on the command line via `-DENABLE_WERROR=ON`. The variable
defaults to NO to not bother developers in their day to day work.

CMakeLists.txt

@@ -147,11 +147,18 @@ ELSE(IS_ABSOLUTE ${INCLUDE_INSTALL_DIR})
 ENDIF(IS_ABSOLUTE ${INCLUDE_INSTALL_DIR})
 
 MACRO(ENABLE_WARNINGS flag)
-	ADD_C_FLAG_IF_SUPPORTED(-W${flag})
+	IF(ENABLE_WERROR)
+		ADD_C_FLAG_IF_SUPPORTED(-Werror=${flag})
+	ELSE()
+		ADD_C_FLAG_IF_SUPPORTED(-W${flag})
+	ENDIF()
 ENDMACRO()
 
 MACRO(DISABLE_WARNINGS flag)
 	ADD_C_FLAG_IF_SUPPORTED(-Wno-${flag})
+	IF(ENABLE_WERROR)
+		ADD_C_FLAG_IF_SUPPORTED(-Wno-error=${flag})
+	ENDIF()
 ENDMACRO()
 
 FUNCTION(TARGET_OS_LIBRARIES target)