getopt: avoid compiler warning during configure

getopt: avoid compiler warning during configure

Some compilers (or warning settings within a compiler) rightfully
complain about assigning a string literal to a char *, since this
can lead to undefined behavior if those characters are modified.
Use the same casts as in the test file to avoid a spurious rejection
of a system getopt merely because of warnings.

* m4/getopt.m4 (gl_GETOPT_CHECK_HEADRS): Avoid problems with
assigning string literals to non-const pointer.

Signed-off-by: Eric Blake <eblake@redhat.com>