pgsql: Suppress compiler warnings about useless comparison of unsigned

Suppress compiler warnings about useless comparison of unsigned to zero.

Reportedly, some compilers warn about tests like "c < 0" if c is unsigned,
and hence complain about the character range checks I added in commit
3bb3f42f3749d40b8d4de65871e8d828b18d4a45. This is a bit of a pain since
the regex library doesn't really want to assume that chr is unsigned.
However, since any such reconfiguration would involve manual edits of
regcustom.h anyway, we can put it on the shoulders of whoever wants to
do that to adjust this new range-checking macro correctly.

Per gripes from Coverity and Andres.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e1df79117a9c3a65b15e9700dd177e4edbae1ad5

Modified Files
--------------
src/backend/regex/regc_lex.c | 6 +++---
src/include/regex/regcustom.h | 11 +++++++++++
2 files changed, 14 insertions(+), 3 deletions(-)