* libabigail.pc.in: New pkgconfig file.
* configure.ac: Define the LIBXML2_VERSION variable that is used
in the libabigail.pc.in file. Generate the libabigail.pc file
from its libabigail.pc.in template.
* Makefile.am: Add libabigail.pc.in to the source distribution.
Install the generated libabigail.pc to the right destination.