Fix building without subordinate IDs support

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Check size of uid_t and gid_t using AC_CHECK_SIZEOF

This built-in check is simpler than the previous method and, most
importantly, works when cross-compiling.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>