New module 'log2f'.

* lib/math.in.h (log2f): New declaration.
* lib/log2f.c: New file.
* m4/log2f.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether log2f is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_LOG2F, HAVE_DECL_LOG2F,
REPLACE_LOG2F.
* modules/math (Makefile.am): Substitute GNULIB_LOG2F, HAVE_DECL_LOG2F,
REPLACE_LOG2F.
* modules/log2f: New file.
* tests/test-math-c++.cc: Check the declaration of log2f.
* doc/posix-functions/log2f.texi: Mention the new module and the IRIX
and OSF/1 and Cygwin problems.