libiberty: Append <libgen.h> to AC_CHECK_DECLS [PR119218].

libiberty: Append <libgen.h> to AC_CHECK_DECLS [PR119218].

Darwin and Solaris, at least, provide basename() in libc, but only
declare it in <libgen.h>.  That library is not one of the set in
AC_INCLUDES_DEFAULT and so we fail the config test and fall back
to the libiberty-provided version.  In itself, this is not an
issue; however, if we include <libgen.h> and libiberty.h in the same
TU we do then get a decl conflict.

	PR other/119218

libiberty/ChangeLog:

	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Append <libgen.h> to AC_INCLUDES_DEFAULT
	when checking for the 'basename' decl.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>