Fix some automake warnings

EXTRA_DIST accepts directories, so just list the testsuite/{success,fail}
directories directly.

Also add some extra AM_PROG_ checks in configure.ac
Move to the parallel-tests test harness

Automake 1.12 uses this harness by default, and fails when
TESTS_ENVIRONMENT is overridden the way we do it. Since Automake 1.11.1
supports it via this option, just add it and drop TESTS_ENVIRONMENT,
since Automake takes care of redirecting the test output now.
Don't treat Automake warnings as errors

Treating warnings as errors helps debugging the build system, but since
Automake changes some behavior generating warnings now and then, it's
safer not to do so and simply watch for warnings.