The cgo tool uses an undocumented environment variable CGOPKGPATH. It was used before the existence of the go tool to tell cgo the full path of the package that it was building, which in turn set the name of the shared library that cgo expected to load back when cgo used shared libraries. CGOPKGPATH no longer does anything useful; it just affects the comments in the generated header file.

We should remove CGOPKGPATH entirely to avoid any future confusion.