pkgconfig: plugin dir should use PKG version not API version.

pkgconfig: plugin dir should use PKG version not API version.

Fix the pluginsdir and includedir variables in the generated pkgconfig
(.pc) files. The location needs to be built with the PKG version in
mind instead of the API version.

While we are at it, also fix the PKG version for GStreamer >= 1.3.

https://bugzilla.gnome.org/show_bug.cgi?id=720820

[additional fixes for includedir and pkg requirements]
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>