Allow autoconf-based clients to detect the library

Allow autoconf-based clients to detect the library

	* configure.ac: Define the components of the version number as
	autoconf variables.  Set the version number to 0.1.0
	* abigail.m4: New file
	* Makefile.am: Add abigail.m4 to the build system.  Install it in
	  $(datadir)/aclocal.
	* src/Makefile.am: Generate and add abg-version.h from the version
	number autoconf variable defined in configure.ac.  Re-generate
	abg-version.h each time configure.ac changes.
	* src/abg-config.h (abigail_get_library_version): Declare ...
	* src/abg-config.cc (abigail_get_library_version): ... and define
	this wirth C linkage.  This is useful for autoconf tests to test
	for the presence of the library.
	* configure: Re-generate.
	* Makefile.in: Likewise.
	* src/Makefile.in: Likewise.
	* tests/Makefile.in: Likewise.