log10f: Work around OSF/1 5.1 bug.

log10f: Work around OSF/1 5.1 bug.

* lib/math.in.h (log10f): Override if REPLACE_LOG10F is 1.
* lib/log10f.c (log10f): If logf exists, use it and provide just the
workaround.
* m4/log10f.m4 (gl_FUNC_LOG10F_WORKS): New macro.
(gl_FUNC_LOG10F): Invoke it. Set REPLACE_LOG10F.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_LOG10F.
* modules/math (Makefile.am): Substitute REPLACE_LOG10F.
* modules/log10f (configure.ac): Consider REPLACE_LOG10F.
(Depends-on): Update conditions.
* doc/posix-functions/log10f.texi: Mention the OSF/1 5.1 problem.