CMake: Major cleanup to support find_package(raylib)

Remove that link_libraries_to_executable() hack and defines a proper
raylib target that can be used with target_link_libraries.

The same target is also available for external (user) code by using
find_package(raylib).

This results in:

- Remove hardcoded build directories from examples and games CMakeLists.txt
- Allow rlgl_standalone and other special examples to be built easily
- Allow CMake projects to find_package(raylib instead of fiddling with pkg-config
- Makes code a little more maintainable
- Fixes #471, #606.
- Makes code less confusing by removing the double use of PLATFORM (#584).

Note that this is still not _The Right Way_(TM), because normally
raylib-config.cmake (or its includes) would be automatically generated.
I didn't manage to get that to work though, so I went the easier route
of just wrapping pkg_check_modules for consumption by find_package.

src/CMakeOptions.txt

@@ -4,7 +4,7 @@ include(EnumOption)
 
 enum_option(PLATFORM "Desktop;Web;Android;Raspberry Pi" "Platform to build for.")
 
-enum_option(OPENGL_VERSION "3.3;2.1;1.1;ES 2.0" "OpenGL Version to build raylib with")
+enum_option(OPENGL_VERSION "OFF;3.3;2.1;1.1;ES 2.0" "Force a specific OpenGL Version?")
 
 # Shared library is always PIC. Static library should be PIC too if linked into a shared library
 option(WITH_PIC "Compile static library as position-independent code" OFF)