Prevent race condition on parallel build

[zeldin]

Two or more threads could all at the same time test that the "work" directory did not exist and then each proceed to create it, with all but one failing. By making it an ordering dependency, this is prevented from happening.

Checklist

Thanks a lot for your contribution! Please tick off the following:

• Tests run successfully (i.e. make test)
  • [x] using the CC65 compiler
  • [x] using the LLVM/Clang compiler
• [x] Doxygen style tags are used for public API
• [x] Source code is properly formatted; run e.g. clang-format -i <file>
• [x] I agree to the GNU Lesser General Public License of this repository

Note: I did not change any code, just the Makefile. I can't run make test currently because I don't have xmega65. Getting that to run is probably a separate adventure. :smile:

[lgblgblgb]

Isn't that more simple to use mkdir -p which won't fail when a directory already exist? Then there is no even need for testing if the directory already exists just mkdir -p work unconditionally.

[zeldin]

Sure, that would work as well.

[zeldin]

I have now managed to build xmega65 for Linux/PPC64 (this turned out to be super easy, kudos!), so all the tests have been run. I also ran the mkdir -p version through the tests and that passed as well. Let me know if you prefer that version and I'll just force push this PR.