Fix up envvar exports for the apt buildpack -- fixes C/C++ compiling.

[kainz]

• 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.

[kainz]

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.

[ddollar]

Thanks :)