Pull Request #533 - Use MAKEFLAGS to pass args to make in all.sh
Modify the script at tests/scripts/all.sh to export the variable
MAKEFLAGS with -j if it was not set before. This should decrease the
total runtime of tests/scripts/all.sh by letting make run multiple jobs
in parallel. Also, add a check at the top of the script to cause a
failure if the environment is not Linux.w