fmodf-ieee: Work around test failure on OSF/1.

fmodf-ieee: Work around test failure on OSF/1.

* m4/fmodf-ieee.m4: New file.
* m4/fmodf.m4 (gl_FUNC_FMODF): If gl_FUNC_FMODF_IEEE is present, test
whether fmodf works with zero arguments. Replace it if not.
* lib/math.in.h (fmodf): Override if REPLACE_FMODF is 1.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_FMODF.
* modules/math (Makefile.am): Substitute REPLACE_FMODF.
* modules/fmodf (configure.ac): Consider REPLACE_FMODF.
(Depends-on): Update dependencies.
* modules/fmodf-ieee (Files): Add m4/fmodf-ieee.m4.
(configure.ac): Invoke gl_FUNC_FMODF_IEEE.
* doc/posix-functions/fmodf.texi: Mention the problem on OSF/1.