pgsql: Fix symlink for errcodes.h so it works in VPATH builds from tarb

Fix symlink for errcodes.h so it works in VPATH builds from tarballs.

backend/Makefile was treating errcodes.h as a header always generated
during build, but actually it's a header provided in tarballs.  Hence,
must use the absolute-symlink recipe, not the relative-symlink one.
Per bug #6072 from Hartmut Raschick.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/25db27808814f474e2080eef8bac068317b7caf6

Modified Files
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src/backend/Makefile |    5 +++--
1 files changed, 3 insertions(+), 2 deletions(-)