Return if another thread already is in compact.
Use AM_TESTS_ENVIRONMENT in Makefile.am so we can safely set TESTS_ENVIRONMENT when running tests (e.g., with valgrind).
Fix compatibility with automake 1.12: we require the parallel test suite, and that wasn't available before 1.12.
Merge pull request #846 from wiredtiger/compact-return

Return if another thread already is in compact.