New module 'modfl'. * lib/math.in.h (modfl): New declaration. * lib/modfl.c: New file. * m4/modfl.m4: New file. * m4/math_h.m4 (gl_MATH_H): Test whether modfl is declared. (gl_MATH_H_DEFAULTS): Initialize GNULIB_MODFL, HAVE_MODFL. * modules/math (Makefile.am): Substitute GNULIB_MODFL, HAVE_MODFL. * modules/modfl: New file. * doc/posix-functions/modfl.texi: Mention the new module.