rpcgen: Generate regress.gen.[c,h] in build rather than src dir Currently an out-of-tree build will either write to the src dir or reuse the existing regress.gen.[c,h]. But if building from a read-only git tree (or if the git dir is cleaned), these files will not exist and the build fails. So write the files to the build dir. If the system does not have python, the regress.gen.[c,h] will be used from the src dir if they exist.