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/LibraryPathToLinkerFlags.cmake

@@ -0,0 +1,24 @@
+function(library_path_to_linker_flags LD_FLAGS LIB_PATHS)
+  foreach(L ${LIB_PATHS})
+    get_filename_component(DIR  ${L} PATH)
+    get_filename_component(LIBFILE ${L} NAME_WE)
+    STRING(REGEX REPLACE "^lib" "" FILE ${LIBFILE})
+
+    if (${L} MATCHES "[.]framework$")
+        set(FILE_OPT "-framework ${FILE}")
+        set(DIR_OPT  "-F${DIR}")
+    else()
+        set(FILE_OPT "-l${FILE}")
+        set(DIR_OPT  "-L${DIR}")
+    endif()
+
+    if ("${DIR}" STREQUAL "" OR "${DIR}" STREQUAL "${LASTDIR}")
+        set (DIR_OPT "")
+    endif()
+
+    set(LASTDIR ${DIR})
+
+    set(${LD_FLAGS} ${${LD_FLAGS}} ${DIR_OPT} ${FILE_OPT} PARENT_SCOPE)
+    string (REPLACE ";" " " ${LD_FLAGS} "${${LD_FLAGS}}")
+  endforeach()
+endfunction()