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.

cmake/Config.cmake

@@ -32,6 +32,14 @@ else()
     return()
 endif()
 
+# set pkgconfig install directory
+# this could be e.g. macports on mac or msys2 on windows etc.
+set(CSFML_PKGCONFIG_DIR "/${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+
+if(SFML_OS_FREEBSD OR SFML_OS_OPENBSD OR SFML_OS_NETBSD)
+    set(CSFML_PKGCONFIG_DIR "/libdata/pkgconfig")
+endif()
+
 # detect the compiler and its version
 # Note: The detection is order is important because:
 # - Visual Studio can both use MSVC and Clang