This is essentially the same as done in 65f2b0a2a3. For spidermonkey_38 I set --enable-posix-nspr-emulation, as it would otherwise complain about a wrong NSPR version and that trick seemed to be successful in spidermonkey_60 anyway.
65f2b0a2a3
- build with `--enable-optimize` - remove unused nspr dependency - cleanup a static library (saves ~20mb)
A new test checks and fails if we're building in js/src, so we create a build dir like Arch does.
