Add pkg-config files, optionally installed by CMake on Linux builds

The modules provided are:

  - csfml-system
  - csfml-graphics
  - csfml-window
  - csfml-audio
  - csfml-network
  - csfml-all (depends on all the above modules)

They are installed to the standard location for pkg-config files on Linux.

This is adapted from SFML own pkg-config files.

tools/pkg-config/csfml-system.pc.in

@@ -0,0 +1,12 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+libdir=${exec_prefix}/@CMAKE_INSTALL_LIBDIR@
+includedir=${prefix}/include
+
+Name: CSFML-system
+Description: The Simple and Fast Multimedia Library for C, system module.
+URL: https://www.sfml-dev.org
+Version: @PROJECT_VERSION@
+Requires.private: sfml-system
+Libs: -L${libdir} -lcsfml-system
+Cflags: -I${includedir}