pgsql: Properly use pg_strcasecmp() instead of strcasecmp().

Log Message:
-----------
Properly use pg_strcasecmp() instead of strcasecmp().
Un-breaks win32 build.

Modified Files:
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pgsql/src/backend/utils/misc:
guc.c (r1.380 -> r1.381)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.380&r2=1.381)