Parallel building randomly fails because bash's Makefiles at some
point do a recursive make invocation to build a file "version.h":
../version.h: ../config.h ../Makefile Makefile
-( cd ${BUILD_DIR} && ${MAKE} ${MFLAGS} version.h )
As a result, both the parent and the child can end up building
version.h at the same time. This causes the build to fail with
mv: cannot stat `newversion.h': No such file or directory
http://hydra.nixos.org/build/3853249