Removed the use of the environment variable PKG_CONFIG_LIBDIR
    
    - Removed the use of the environment variable PKG_CONFIG_LIBDIR
      While building binaries the tool {i686,x86_64}-w64-mingw32-pkg-config
      should be used to find out pkg-config information
      The environment variable PKG_CONFIG already automatically points
      to the right cross-compiler aware version of pkg-config when
      the mingw{32,64}-pkg-config packages are installed
    - Fixes compilation of mingw-gtk3 3.9.0 (GNOME BZ #699690)
    - Automatically add R: mingw{32,64}-pkg-config tags when .pc files
      are detected while building mingw packages