* lib/math.in.h (logbf): New declaration.
* lib/logbf.c: New file.
* m4/logbf.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether logbf is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_LOGBF, HAVE_LOGBF,
REPLACE_LOGBF.
* modules/math (Makefile.am): Substitute GNULIB_LOGBF, HAVE_LOGBF,
REPLACE_LOGBF.
* modules/logbf: New file.
* tests/test-math-c++.cc: Check the declaration of logbf.
* doc/posix-functions/logbf.texi: Mention the new module.