logl: Work around OSF/1 5.1 bug.

logl: Work around OSF/1 5.1 bug.

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