segment.

The API will use this counter to check for abandonment.

Make use of the SHM age attribute in VSM_Abandoned.

This gives a less expensive way (without doing stat syscall) to check
for abandonment. If there's been more than 2 seconds between calls to
VSM_Abandoned and the age attribute hasn't changed in between, the
stat on the file will be performed to see if the SHM file has changed.

VSM_Abandoned will be the standard way to check for SHM changes in API
utilities, and when it returns true, the application is supposed to
call VSM_Close and then VSM_Open to reopen the SHM file.

Update Makefile.am in various places for build