pgsql: We don't need to rebuild objfiles.txt every time an object file

Log Message:
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We don't need to rebuild objfiles.txt every time an object file changes.
So only rebuild when a makefile changes (which presumably defines the
file list somewhere), and only touch the file if an object changed. The
touch is necessary so the parent make knows something changed and
ultimately rebuilds postgres.

Modified Files:
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pgsql/src/backend:
common.mk (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/common.mk?r1=1.2&r2=1.3)