configure.ac: Check for strtok_r too

In order to not try compiling our own version of strtok_r, which can
potentially conflict with the libc-provided one, check for strtok_r in
configure.ac, so that the HAVE_STRTOK_R gets defined correctly where
strtok_r is available.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>