ecpg: Improve test building

Further improve on commit c75e1436.
Instead of building both .o files and binaries in the same make rule,
just rely on the normal .c -> .o rule.  This will ensure that
dependency tracking is used when enabled.  To do this, disable the
implicit direct .c -> binary rule globally, which will also prevent
the original problem (*.dSYM junk) from reappearing elsewhere.