New module 'log10f'. * lib/math.in.h (log10f): New declaration. * lib/log10f.c: New file. * m4/log10f.m4: New file. * m4/math_h.m4 (gl_MATH_H): Test whether log10f is declared. (gl_MATH_H_DEFAULTS): Initialize GNULIB_LOG10F, HAVE_LOG10F. * modules/math (Makefile.am): Substitute GNULIB_LOG10F, HAVE_LOG10F. * modules/log10f: New file. * tests/test-math-c++.cc: Check the declaration of log10f. * doc/posix-functions/log10f.texi: Mention the new module.