Fix the disablement of pgrefdebugging

Fix the disablement of pgrefdebugging

Currently, if you add the flag --disable-pgrefdebugging during configure
it enables pgrefdebugging. The only way for it to actually be disabled
is to remove the flag entirely. This patch makes the flag respect the
--enable and --disable options correctly. Therefore,
--enable-pgrefdebugging is the only way to enable this now and
--enable-pgrefdebugging=no and --disable-pgrefdebugging work as
expected.

Signed-off-by: William A. Kennington III <william@wkennington.com>