pgsql: Improve efficiency of recent changes to plperl's sv2cstr().

Improve efficiency of recent changes to plperl's sv2cstr().

Along the way, add a missing dependency in the GNUmakefile.

Alex Hunsaker, with a slight adjustment by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/01d83ffdcae92f75dbfd41de0b4213d241edd394

Modified Files
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src/pl/plperl/GNUmakefile | 4 ++--
src/pl/plperl/expected/plperl_elog.out | 4 ++++
src/pl/plperl/plperl_helpers.h | 21 ++++++++++++++-------
src/pl/plperl/sql/plperl_elog.sql | 3 +++
4 files changed, 23 insertions(+), 9 deletions(-)