New module 'frexpf'.

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