Fix build parallelism in 'pd-externals' module
I made a mistake in commit 4f95916a which caused the build to run with `make -j`, so it would start a huge number of compile commands in parallel and eventually abort. Fixing this caused the 'pd-externals' build to run mostly with `make -j 1`, so it would run one command at a time and go very slowly. I have patched the Makefile with the necessary '+' prefix for recursive `make` invocations so that things now work as expected.
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