bash: Disable parallel building

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