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.cmake

@@ -0,0 +1,68 @@
+# - Try to find raylib
+# Options:
+#   raylib_USE_STATIC_LIBS - OFF by default
+#   raylib_VERBOSE         - OFF by default
+# Once done, this defines a raylib target that can be passed to
+# target_link_libraries as well as following variables:
+#
+#   raylib_FOUND - System has raylib installed
+#   raylib_INCLUDE_DIRS - The include directories for the raylib header(s)
+#   raylib_LIBRARIES - The libraries needed to use raylib
+#   raylib_LDFLAGS - The linker flags needed with raylib
+#   raylib_DEFINITIONS - Compiler switches required for using raylib
+
+set(XPREFIX PC_RAYLIB)
+if (raylib_USE_STATIC_LIBS)
+    set(XPREFIX ${XPREFIX}_STATIC)
+endif()
+
+find_package(PkgConfig)
+pkg_check_modules(${XPREFIX} REQUIRED raylib)
+set(raylib_DEFINITIONS ${${XPREFIX}_CFLAGS})
+
+find_path(raylib_INCLUDE_DIR
+    NAMES raylib.h
+    HINTS ${${XPREFIX}_INCLUDE_DIRS}
+)
+
+find_library(raylib_LIBRARY
+    NAMES raylib
+    HINTS ${${XPREFIX}_LIBRARY_DIRS}
+)
+
+set(raylib_LIBRARIES    ${raylib_LIBRARY})
+set(raylib_LIBRARY_DIRS ${${XPREFIX}_LIBRARY_DIRS})
+set(raylib_LIBRARY_DIR  ${raylib_LIBRARY_DIRS})
+set(raylib_INCLUDE_DIRS ${raylib_INCLUDE_DIR})
+set(raylib_LDFLAGS      ${${XPREFIX}_LDFLAGS})
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(raylib DEFAULT_MSG
+    raylib_LIBRARY
+    raylib_INCLUDE_DIR
+)
+
+mark_as_advanced(raylib_LIBRARY raylib_INCLUDE_DIR)
+
+if (raylib_USE_STATIC_LIBS)
+  add_library(raylib STATIC IMPORTED GLOBAL)
+else()
+  add_library(raylib SHARED IMPORTED GLOBAL)
+endif()
+string (REPLACE ";" " " raylib_LDFLAGS "${raylib_LDFLAGS}")
+
+set_target_properties(raylib
+  PROPERTIES
+  IMPORTED_LOCATION             "${raylib_LIBRARIES}"
+  INTERFACE_INCLUDE_DIRECTORIES "${raylib_INCLUDE_DIRS}"
+  INTERFACE_LINK_LIBRARIES      "${raylib_LDFLAGS}"
+  INTERFACE_COMPILE_OPTIONS     "${raylib_DEFINITIONS}"
+)
+
+if (raylib_VERBOSE)
+  message(STATUS "raylib_FOUND: ${raylib_FOUND}")
+  message(STATUS "raylib_INCLUDE_DIRS: ${raylib_INCLUDE_DIRS}")
+  message(STATUS "raylib_LIBRARIES: ${raylib_LIBRARIES}")
+  message(STATUS "raylib_LDFLAGS: ${raylib_LDFLAGS}")
+  message(STATUS "raylib_DEFINITIONS: ${raylib_DEFINITIONS}")
+endif()