New module 'log2l'.
* lib/math.in.h (log2l): New declaration.
* lib/log2l.c: New file.
* m4/log2l.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether log2l is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_LOG2L, HAVE_DECL_LOG2L,
REPLACE_LOG2L.
* modules/math (Makefile.am): Substitute GNULIB_LOG2L, HAVE_DECL_LOG2L,
REPLACE_LOG2L.
* modules/log2l: New file.
* tests/test-math-c++.cc: Check the declaration of log2l.
* doc/posix-functions/log2l.texi: Mention the new module and the IRIX
and OSF/1 problems.