Allow building of cross rubies in parallel (#169)
Allow building of cross rubies in parallel (#169)

Rubies can be build like so:
  rake-compiler cross-ruby VERSION=2.7.0:2.6.0 HOST=x86_64-w64-mingw32:i686-w64-mingw32

This builds the cross product of all ":" separated ruby and host versions.
To force sequential builds add option "-j1".
rake-compiler/rake-compiler