At least in the case of cmake, we can build jsc out of tree if building with cmake directly, but build-jsc does not allow it.
Created attachment 329350 [details]
Patch fixing the issue for cmake. I don't know how to fix this for other build systems and don't have a way of testing them.
Comment on attachment 329350 [details]
Clearing flags on attachment: 329350
Committed r226225: <https://trac.webkit.org/changeset/226225>
All reviewed patches have been landed. Closing bug.