(I'll address the parallel build part in addition to what has been said, but please respond to my email address/sourcehut mailing list for any further communication on these issues).
I should have communicated this more clearly in README. Kefir requires relatively recent GNU Make versions (see dist/Dockerfile for full and complete set of dependencies), as it basically uses make for all project lifecycle tasks. I remember there were some parallel build issues with older GNU Make versions. This could be an issue.
For out-of-tree build, use KEFIR_BIN_DIR make parameter. I use it quite extensively in https://git.sr.ht/~jprotopopov/kefir/tree/master/item/script... so this should work.
DESTDIR is supported as a parameter of make install: https://git.sr.ht/~jprotopopov/kefir/tree/master/item/instal...
As for the parallel build issues, contact me over the email please