CMake: Add ENABLE_WARNINGS and ENABLE_WERROR build options

ENABLE_WARNINGS is enabled by default, with `-Wall` on GCC/Clang and
`/W4` on MSVC.

ENABLE_WERROR is disabled by default, treats warnings as errors.

desktop_version/CMakeLists.txt

@@ -3,6 +3,10 @@
 
 CMAKE_MINIMUM_REQUIRED(VERSION 2.8.12)
 
+# CMake Options
+OPTION(ENABLE_WARNINGS "Enable compilation warnings" ON)
+OPTION(ENABLE_WERROR "Treat compilation warnings as errors" OFF)
+
 # Architecture Flags
 IF(APPLE)
 	# Wow, Apple is a huge jerk these days huh?
@@ -103,6 +107,17 @@ ELSE()
 	ADD_EXECUTABLE(vvvvvv ${VVV_SRC})
 ENDIF()
 
+# Build options
+IF(ENABLE_WARNINGS)
+	# The weird syntax is due to CMake generator expressions.
+	# Saves quite a few lines and boilerplate at the price of readability.
+	TARGET_COMPILE_OPTIONS(vvvvvv PRIVATE
+		$<$<OR:$<CXX_COMPILER_ID:GNU>,$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
+			-Wall $<$<BOOL:ENABLE_WERROR>:-Werror>>
+		$<$<CXX_COMPILER_ID:MSVC>:
+			/W4 $<$<BOOL:ENABLE_WERROR>:/WX>>)
+ENDIF()
+
 # Library information
 ADD_LIBRARY(tinyxml-static STATIC ${XML_SRC})
 ADD_LIBRARY(physfs-static STATIC ${PFS_SRC} ${PFSP_SRC})