logf: Work around OSF/1 5.1 bug.

logf: Work around OSF/1 5.1 bug.

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