Probe for shm* routines in configure.com.

They became available in ACRTL ECO V0100 on VMS v8.4, but aren't
quite ready for prime time yet.  Specifically, shmget fails
unless IPC_CREAT is set in the third argument, but Perl has tests
that explicitly check that this is not necessary.

So construct the probe such that we won't enable these routines
until the CRTL bug has been fixed.