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.