Allow run make check of PostgreSQL with loading pg_hint.plan

Allow run make check of PostgreSQL with loading pg_hint.plan

Automatically switch environment when pg_hint_plan is installed as a
part of "make check" of PostgreSQL.  That test is executed with the
following command line in the top directory of PostgreSQL tree.

make check EXTRA_INSTALL=<this directory> EXTRA_REGRESS_OPTS="--temp-config <this directory>/pg_hint_plan.conf"

make check-world fails for test that requires extra modules.