M #-: Various Makefile fixes

- Use more common () instead of {} (which can be confusing when shell
  variables are used)
- Use :%= idiom instead of patsubst (takes much less space)
- Make use of (WINDOWS/LINUX)_CONTEXT_PACKAGES_FULL variables (reduce
  visual complexity)
- Add .SECONDEXPANSION to handle conditional windows/linux context
  builds
- Make windows/linux context builds fully conditional
- Use correct versioning for windows context artifacts
- Make sure "rm -rf /*" is extra impossible to execute (paranoid)
- Fix help message for windows targets
- Remove unneeded whitespace + convert some "insignificant" tabs to spaces