(AC_FUNC_STRERROR_R): Always do char* test, so that it gets cached.
Merge changes from latest Autoconf CVS.
Rename ac_cv_func_strerror_r_works to ac_cv_func_strerror_r_char_p,
and rename HAVE_WORKING_STRERROR_R to STRERROR_R_CHAR_P, since
POSIX decided to standardize on the int flavor of strerror_r.