Fix libtool builds

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26452 f0d54148-0727-0410-94bb-9a71ac55c965
Add pkg-config gdal.pc (adapted from mloskot's patch, #3470)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26453 f0d54148-0727-0410-94bb-9a71ac55c965