strtoumax: Avoid link error on OSF/1 with DTK cc.

* m4/strtoumax.m4 (gl_FUNC_STRTOUMAX): Always test whether strtoumax is
defined as a function.
* modules/strtoumax (Depends-on, configure.ac): Test only whether
strtoumax is defined, not whether it is declared.
(cherry picked from commit 24bfc89ccc3fd4ce86652dbbb95afd844db59a07)