Build specific example using -DBUILD_EXAMPLE Cmake flag

[benjitrosch]

As the title says, this allows users to build only one example rather than all of them using CMake by passing a BUILD_EXAMPLE flag:

cmake -B build -DBUILD_EXAMPLE=models_loading_gltf

Making a change to the core Raylib library will cause all of the examples to rebuild which can take a long time. This would be a helpful timesaver to contributors who use CMake through the terminal, or with an IDE like VSCode or Neovim etc. rather than using the Visual Studio solution.

[raysan5]

@benjitrosch Nice improvement! Also supported by default Makefile.

[Peter0x44]

I can't help but feel there has to be a better way to implement this in cmake

[benjitrosch]

@Peter0x44 what did you have in mind?

[Peter0x44]

This commit is not needed, and probably should be reverted.
This already works likeso:

$ cmake --build . -t core_basic_window
[ 85%] Built target glfw
[ 85%] Building C object raylib/CMakeFiles/raylib.dir/rtextures.c.o
[ 85%] Linking C static library libraylib.a
[100%] Built target raylib
[100%] Linking C executable core_basic_window
[100%] Built target core_basic_window

You can pick a specific target and it will rebuild only that target and its dependencies.
This option is not necessary or useful.

[raysan5]

@Peter0x44 Oh! Thanks for investigating this improvement!