modfl-ieee: Work around test failures on IRIX, OSF/1, mingw.

modfl-ieee: Work around test failures on IRIX, OSF/1, mingw.

* m4/modfl-ieee.m4: New file.
* m4/modfl.m4 (gl_FUNC_MODFL): If gl_FUNC_MODFL_IEEE is present, test
whether modfl works with Inf. Replace it if not.
* lib/math.in.h (modfl): Override if REPLACE_MODFF is 1.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_MODFL.
* modules/math (Makefile.am): Substitute REPLACE_MODFL.
* modules/modfl (configure.ac): Consider REPLACE_MODFL.
(Depends-on): Update dependencies.
* modules/modfl-ieee (Files): Add m4/modfl-ieee.m4, m4/minus-zero.m4,
m4/signbit.m4.
(configure.ac): Invoke gl_FUNC_MODFL_IEEE.
* doc/posix-functions/modfl.texi: Mention the modfl-ieee module.