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

@@ -49,6 +49,7 @@ OPTION( VALGRIND			"Configure build for valgrind"			OFF )
 OPTION( CURL			"Use curl for HTTP if available" ON)
 OPTION( USE_EXT_HTTP_PARSER		"Use system HTTP_Parser if available" ON)
 OPTION( DEBUG_POOL			"Enable debug pool allocator"			OFF )
+OPTION( ENABLE_WERROR			"Enable compilation with -Werror"		OFF )
 
 IF(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
 	SET( USE_ICONV ON )
@@ -224,11 +225,18 @@ ELSE ()
 	SET(CMAKE_C_FLAGS "-D_GNU_SOURCE ${CMAKE_C_FLAGS}")
 
 	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()
 
 	ENABLE_WARNINGS(all)