How can I make the Python configure build system avoid refreshing source code that it’s already downloaded? When configuring PETSc, and indirectly building scalapack from PETSc (because of the --download-scalapack flag) I’m getting an error in MacOS because the ${objslamov} objects in Makefile.parallel from scalapack are missing the -isysroot flag pointing to the MacOS SDK (while the rest of objects in Makefile.parallel are keeping the flag). The fix is shockingly simple (just add the -isysroot flag to Makefile.parallel and then scalapack builds successfully), but then when I try to rerun the PETSc configure line from your tutorial, it cleans scalapack, downloads it again, and of course it fails 
Is there any way for telling the Python configure not to redownload a package and just use the version already downloaded?