since out makefiles depend on the mtime of the built thirdparty
libraries, changing their mtime results in a relink of userspace
components. their mtime is changed by the cp done from the thirdparty
makefile, which is not necessary if the files themselves have not
changed. symlinking instead of copying the libraries solves this neatly.
(cherry picked from commit 52d38528e008501f5fa1923520e284fb07132b1e)