Avoid adding extra : characters to the end of gcc paths. This will
cause gcc to use the current directory in system include paths, which
breaks C #include vs #include "file" semantics.
Also, write an export file for buildpack-multi to pass the exported
envvars to the next buildpack.
So there was a bit where I was not doing bits right that were supposed to agree with BUILD_DIR in some other buildpack-using environments. This should work correctly everywhere again.