Log Message:
databases/postgresql91(-client): Fix build for gcc46 and gcc47

Postgresql91 uses non-constant array sizes in record definitions which
gcc enforces starting with 4.6.  These index sizes are defined as macros
using functions such as offsetof.  These patches introduce enums where
the macros become constant expressions which gcc 4.6+ will accept.

GCC 4.7 also introduces the unused-but-set-variable warning which is
popping up all over the place in pgsql91, so silence these warnings -
they are harmless and get optimized out anyway.

No revbump because functionality won't change on binaries generated with
gcc4.5 and below.