Ensuring that all dirs in LDFLAGS exist is only needed for Ruby due to its `configure` requirements.
If some LDFLAGS entries point to a nonexisting path to which the user doesn't have permission. this causes a build failure.
The current checksums for tar.xz and .tgz files are reversed. This
causes python-build to fail with a checksum mismatch error.
Reversing the checksums to allow the build to succeed.