Port better to AIX.

Port better to AIX.

* configure.ac (CFLAGS): Append -O if the user did not specify CFLAGS,
we did not already infer an optimization option, and -O works.
AIX xlc needs -O, otherwise garbage collection doesn't work.
* src/lisp.h (ENUM_BF) [__IBMC__]: Make it 'unsigned int' here, too,
to pacify AIX xlc.