better place to handle dependencies.
Make another step at helping cross-compiling: when the user specifies
an alternate source tree, the script takes care of creating include
files and libraries for the new tree.
Furthermore, build and use a version of the "config" program which
matches the new sources.
It takes a long time to create libraries, and it might even not do
the right thing at once, there might be some dependencies that i
have forgotten. At any rate, with this code i have been able to
build a working picobsd image using -CURRENT sources on -STABLE
MFC after: 3 days
a39c93813c