SpiderMonkey 60

Upstream tarballs can now be used without a laundry list of patches, so
build from an upstream tarball, unlike mozjs52 which builds from my git
repository. (Mozilla are not yet publishing their tarballs though, so
for now the tarball still lives in releng.)

See GNOME/gjs#161.
1 job for mozjs60 in 19 seconds (queued for 49 minutes and 31 seconds)