pgsql: Use the preferred version of xsubpp, not necessarily the one tha

Use the preferred version of xsubpp, not necessarily the one that came with the
distro version of perl.

David Wheeler and Alex Hunsaker.

Backpatch to 9.1 where it applies cleanly. A simple workaround is available for earlier
branches, and further effort doesn't seem warranted.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/403372459efe4c938d6777e9c10cadc2811c835a

Modified Files
--------------
src/pl/plperl/GNUmakefile | 7 +++++--
src/tools/msvc/Mkvcbuild.pm | 5 ++++-
2 files changed, 9 insertions(+), 3 deletions(-)