truncl-ieee: Make it work on OSF/1 5.1 with cc.

* doc/posix-functions/truncl.texi: Mention the OSF/1 5.1 bug.
* m4/truncl.m4 (gl_FUNC_TRUNCL): If gl_FUNC_TRUNCL_IEEE is also used,
test whether truncl works according to ISO C 99 with IEC 60559.
* m4/truncl-ieee.m4: New file.
* modules/truncl-ieee (Files): Add it and m4/minus-zero.m4,
m4/signbit.m4.
(configure.ac): Invoke gl_FUNC_TRUNCL_IEEE.