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/raylib-config-version.cmake

@@ -0,0 +1,21 @@
+set(PACKAGE_VERSION "@PROJECT_VERSION@")
+
+if(PACKAGE_FIND_VERSION VERSION_EQUAL PACKAGE_VERSION)
+  set(PACKAGE_VERSION_EXACT TRUE)
+endif()
+if(NOT PACKAGE_FIND_VERSION VERSION_GREATER PACKAGE_VERSION)
+  set(PACKAGE_VERSION_COMPATIBLE TRUE)
+else(NOT PACKAGE_FIND_VERSION VERSION_GREATER PACKAGE_VERSION)
+  set(PACKAGE_VERSION_UNSUITABLE TRUE)
+endif(NOT PACKAGE_FIND_VERSION VERSION_GREATER PACKAGE_VERSION)
+
+# if the installed or the using project don't have CMAKE_SIZEOF_VOID_P set, ignore it:
+if("${CMAKE_SIZEOF_VOID_P}" STREQUAL "" OR "@CMAKE_SIZEOF_VOID_P@" STREQUAL "")
+   return()
+endif()
+
+if(NOT CMAKE_SIZEOF_VOID_P STREQUAL "@CMAKE_SIZEOF_VOID_P@")
+  math(EXPR installedBits "8 * 8")
+  set(PACKAGE_VERSION "${PACKAGE_VERSION} (${installedBits}bit)")
+  set(PACKAGE_VERSION_UNSUITABLE TRUE)
+endif()