Make pqsignal() available to pg_regress of ECPG and isolation suites.

Commit 453a5d91 made it available to the
src/test/regress build of pg_regress, but all pg_regress builds need the
same treatment.  Patch 9.2 through 8.4; in 9.3 and later, pg_regress
gets pqsignal() via libpgport.