"make check" should detect which PostgreSQL instance to use from pg_config

[kneufeld]

Currently "make check" requires createdb, psql, and dropdb to be on the user's path. If more than one instance of PostgreSQL has been installed on a system this is problematic.

configure already detected which PostgreSQL instance to build against … why not prefix the binaries with BINDIR found in pg_config rather than relying on what may or may not exist in one's PATH.

[pramsey]

Patch in trunk at r4131