Add pkg-config check for SDL lib and fallback to deprecated sdl2-config

[kea]

The use of sdl2-config was deprecated here https://github.com/libsdl-org/SDL/pull/6467

I also moved the main extension C file and header to the project's root directory to permit the extension's build in the php-src/ext directory. The is a limitation on the PHP build system that includes only the H file from the root of the extensions. https://github.com/php/php-src/blob/2d883a6ff09bfc588e5d714a392a1f0e89a09926/build/genif.sh#L39