* lib/string.in.h (sigdescr_np): New declaration.
* lib/sigdescr_np.c: New file.
* m4/sigdescr_np.m4: New file.
* m4/string_h.m4 (gl_HEADER_STRING_H_BODY): Test whether sigdescr_np is
declared.
(gl_HEADER_STRING_H_DEFAULTS): Initialize GNULIB_SIGDESCR_NP,
HAVE_SIGDESCR_NP.
* modules/string (Makefile.am): Substitute GNULIB_SIGDESCR_NP,
HAVE_SIGDESCR_NP.
* modules/sigdescr_np: New file.
* tests/test-string-c++.cc: Verify the signature of sigdescr_np.
* doc/glibc-functions/sigdescr_np.texi: Mention the new module.