One common problem when dealing with shared library is
	keeping headers and libraries synchronized. Hard to debug
	problems can be triggered by the inclusion of old headers.

	The approach implemented in this patch keeps two strings
	containing the package version: one is the header and
	one compiled in the library. A macro is also provided
	to allow the comparison of both values. This can be used
	in libraries depending on hpp-util to make sure the library
	is recent enough.

	* configure.ac: Add dependency toward Boost.String.
	* include/hpp/util/version.hh.in: New.
	* src/Makefile.am: Compile new source file.
	* src/version.cc: New.

Signed-off-by: Thomas Moulard <thomas.moulard@gmail.com>