makefile feature request: make clean-except-for-external

[Noel Grandin]

Hi

I'm doing a lot of work with clang plugins, so I often need to build
from scratch, which takes a really long time.

It would be awesome if I could do a
  make clean-except-for-externals
which would delete all of the build output except for the stuff we
generate from the externals directory,
for which we do not run the clang plugins.

Thanks, Noel Grandin