with bytecode and jit
mostly on travis
This only happens with make -j>1 v3.81, but not with -j0 and neither with gmake-4.0, even -j4.
Cannot be reproduced with pure potion or any shell script.
external make-3.81 bug: fixed in gmake-4.0
try to workaround issue #53, a make-3.81 bug with parallel make.
worth a try just to fool travis