Support pkgconfig through a new libabigail.pc file

* 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.

Makefile.am

@@ -5,6 +5,10 @@ headers = config.h
 m4datadir = $(datadir)/aclocal
 m4data_DATA = $(srcdir)/abigail.m4
 
-EXTRA_DIST = \
-README INSTALL COPYING \
-$(headers) $(m4data_DATA)
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = libabigail.pc
+
+EXTRA_DIST = 			\
+README INSTALL COPYING 		\
+$(headers) $(m4data_DATA) 	\
+libabigail.pc.in

configure.ac

@@ -43,6 +43,8 @@ AC_LANG_COMPILER_REQUIRE
 dnl Check for dependency: libxml
 LIBXML2_VERSION=2.6.22
 PKG_CHECK_MODULES(XML, libxml-2.0 >= $LIBXML2_VERSION)
+
+AC_SUBST(LIBXML2_VERSION)
 AC_SUBST(XML_LIBS)
 AC_SUBST(XML_CFLAGS)
 
@@ -54,6 +56,7 @@ if test x$ABIGAIL_DEVEL != x; then
 fi
 
 AC_CONFIG_FILES([Makefile
+libabigail.pc
 include/Makefile
 doc/Makefile
 src/Makefile

libabigail.pc.in

@@ -0,0 +1,11 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: libabigail
+Description: The ABI Generic Analysis and Instrumentation Library
+Version: @VERSION@
+Requires: libxml-2.0 >= @LIBXML2_VERSION@
+Libs: -L${libdir} -labigail
+Cflags: -I${includedir}/libabigail