New module 'ilogbf'.

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