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.

cmake/PopulateConfigVariablesLocally.cmake

@@ -0,0 +1,11 @@
+macro(populate_config_variables_locally target)
+  get_property(raylib_INCLUDE_DIRS TARGET ${target} PROPERTY INTERFACE_INCLUDE_DIRECTORIES)
+  #get_property(raylib_LIBRARIES    TARGET ${target} PROPERTY LOCATION) # only works for SHARED
+  get_property(raylib_LDFLAGS      TARGET ${target} PROPERTY INTERFACE_LINK_LIBRARIES)
+  get_property(raylib_DEFINITIONS  TARGET ${target} PROPERTY DEFINITIONS)
+
+  set(raylib_INCLUDE_DIRS "${raylib_INCLUDE_DIRS}" PARENT_SCOPE)
+  #set(raylib_LIBRARIES   "${raylib_INCLUDE_DIRS}" PARENT_SCOPE)
+  set(raylib_LDFLAGS      "${raylib_LDFLAGS}" PARENT_SCOPE)
+  set(raylib_DEFINITIONS  "${raylib_DEFINITIONS}" PARENT_SCOPE)
+endmacro()