More build script generality in finding btree.[ch].

I now foresee those files ending up directly in the Tweak main source
tree, so I should ensure the build script can cope with that
possibility as well as the other two it currently handles.


git-svn-id: svn://svn.tartarus.org/sgt@10273 cda61777-01e9-0310-a592-d414129be87e