New module 'ilogbl'.

* lib/math.in.h (ilogbl): New declaration.
* lib/ilogbl.c: New file.
* m4/ilogbl.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether ilogbl is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_ILOGBL, HAVE_ILOGBL.
* modules/math (Makefile.am): Substitute GNULIB_ILOGBL, HAVE_ILOGBL.
Split sed invocation, to avoid the limit of 100 substitutions of
HP-UX 'sed'.
* modules/ilogbl: New file.
* tests/test-math-c++.cc: Check the declaration of ilogbl.
* doc/posix-functions/ilogbl.texi: Mention the new module.