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