make -j clean all doesn't work every time

make -j 3 clean all sometimes fail.

This system has two cores, running FreeBSD 8.2 and autoconf-2.69, automake-1.12, libtool-2.4.2

These are the failures, in order of frequency. The first failure is the most common by far. In total, I seem to have a failure rate of about 60%.

$ make -j 3 clean all

CCLD src/test/test gcc: src/test/src_test_test-test_data.o: No such file or directory gcc: src/test/src_test_test-tinytest.o: No such file or directory

CCLD src/or/tor gcc: src/or/tor_main.o: No such file or directory

CCLD src/test/test gcc: src/test/src_test_test-test_data.o: No such file or directory

Related to #6524 (moved).