Allow for conditional building of Steam and GOG APIs

I think it's a bit silly to always include the Steam and GOG APIs
whenever we build VVVVVV, since the only time they'll ever be used is in
a live build and not a dev build.

So now Steam and GOG are disabled by default. If you want them, you'll
need to add -DSTEAM=ON or -DGOG=ON respectively at CMake time. They're
also both automatically enabled for release builds.

desktop_version/CMakeLists.txt

@@ -10,6 +10,14 @@ OPTION(ENABLE_WERROR "Treat compilation warnings as errors" OFF)
 SET(CUSTOM_LEVEL_SUPPORT ENABLED CACHE STRING "Optionally disable playing and/or editing of custom levels")
 SET_PROPERTY(CACHE CUSTOM_LEVEL_SUPPORT PROPERTY STRINGS ENABLED NO_EDITOR DISABLED)
 
+SET(STEAM OFF CACHE BOOL "Use the Steam API")
+SET(GOG OFF CACHE BOOL "Use the GOG API")
+
+IF(CMAKE_BUILD_TYPE STREQUAL "Release" OR CMAKE_BUILD_TYPE STREQUAL "RelWithDebInfo" OR CMAKE_BUILD_TYPE STREQUAL "MinSizeRel")
+	SET(STEAM ON)
+	SET(GOG ON)
+ENDIF()
+
 # Architecture Flags
 IF(APPLE)
 	# Wow, Apple is a huge jerk these days huh?
@@ -95,12 +103,18 @@ SET(VVV_SRC
 	src/WarpClass.cpp
 	src/main.cpp
 	src/Network.c
-	src/SteamNetwork.c
-	src/GOGNetwork.c
 )
 IF(NOT CUSTOM_LEVEL_SUPPORT STREQUAL "DISABLED")
 	LIST(APPEND VVV_SRC src/editor.cpp)
 ENDIF()
+IF(STEAM)
+	LIST(APPEND VVV_SRC src/SteamNetwork.c)
+	ADD_DEFINITIONS(-DSTEAM_NETWORK)
+ENDIF()
+IF(GOG)
+	LIST(APPEND VVV_SRC src/GOGNetwork.c)
+	ADD_DEFINITIONS(-DGOG_NETWORK)
+ENDIF()
 
 SET(XML_SRC
 	../third_party/tinyxml/tinystr.cpp