cmake: do not prefer system libs by default

Engine or game submodules maybe much recent than the one the system provides,
and define symbols not found in system libraries.

While it is probably unlikely to happen with freetype,
this is more likely to happen with some game libraries like Lua.

Author: illwieckz

CMakeLists.txt

@@ -766,7 +766,7 @@ if (USE_BREAKPAD)
     endif()
 endif()
 
-option(PREFER_EXTERNAL_LIBS "Tries to use system libs where possible." ON)
+option(PREFER_EXTERNAL_LIBS "Tries to use system libs where possible." OFF)
 
 macro(prefer_package LIB_NAME LIB_CMAKE)
 	if (NOT ${LIB_NAME}_FOUND)