Fix #908: allow lazy-compiling from several threads at once (compilation is serialized)
Remove indirection, and add comment about why the signature may already have been compiled for
Add basic tests for non-reentrant locks
Merge branch 'master' of github.com:numba/numba into fix908
Merge pull request #910 from pitrou/fix908

Fix #908: allow lazy-compiling from several threads at once