AC_FUNC_STRERROR_R: Include string.h in test program.

AC_FUNC_STRERROR_R: Include string.h in test program.

I misremembered how AC_LANG_PROGRAM works.  We don’t need to invoke
AC_INCLUDES_DEFAULT here but we *do* need to explicitly include
string.h.

Unfortunately we have no good way of testing for this regression with
the testsuite as it is today.

* lib/autoconf/functions.m4 (AC_FUNC_STRERROR_R): Include string.h in
  test program.