New module 'log2'.

New module 'log2'.

* lib/math.in.h (log2): New declaration.
* lib/log2.c: New file.
* m4/log2.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether log2 is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_LOG2, HAVE_DECL_LOG2,
REPLACE_LOG2.
* modules/math (Makefile.am): Substitute GNULIB_LOG2, HAVE_DECL_LOG2,
REPLACE_LOG2.
* modules/log2: New file.
* tests/test-math-c++.cc: Check the declaration of log2.
* doc/posix-functions/log2.texi: Mention the new module and the IRIX
and OSF/1 and Cygwin problems.