To stop the errors when building concurrently, we make a number of
changes:
1. stop including generated files in *_SOURCES,
2. make *-protos.h and *-private.h depend on the *_SOURCES,
3. make all objects depend on *-{protos,private}.h,
4. in a few places change dir/header.h to $(srcdir)/dir/header.h,
This appears to work for me with make -j16 on a 4-way box.
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