New module 'fabsf'. * lib/math.in.h (fabsf): New declaration. * lib/fabsf.c: New file. * m4/fabsf.m4: New file. * m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize GNULIB_FABSF, HAVE_FABSF. * modules/math (Makefile.am): Substitute GNULIB_FABSF, HAVE_FABSF. * modules/fabsf: New file. * tests/test-math-c++.cc: Check the declaration of fabsf. * doc/posix-functions/fabsf.texi: Mention the new module.