Prepare support for symbol visibility control

Prepare support for symbol visibility control

While working on something else, I started to prepare the support for
visibility control in the library.  This patch detects that we are
using a compiler that supports GCC's visibility control attributes and
defines macros ABG_HIDDEN, ABG_EXPORTED,
ABG_{BEGIN,END}_EXPORT_DECLARATIONS to control the visibility of
declarations accordingly.  These are not yet used but using them
should now be easy.

	* configure.ac: Detect compiler support for
	__attribute__((visibility("hidden")) and define the
	HAS_GCC_VISIBILITY_ATTRIBUTE macro accordingly.  Update the
	configuration report.
	* src/abg-internal.h: New internal header file that defines macros
	to be used in the source code to control declaration visibility.
	* src/Makefile.am: Add abg-internal.h to source distribution.  Add
	src/ to the include search path.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>