Update instructions for manually building 7zip

With #41527 (closed) dmg files are now compressed with LZMA, and the tools/dmg2mar script requires a recent version of 7z to be able to extract the dmg file.

If the distribution used does not provide a recent version, the script includes some instructions for manually building p7zip, however those instructions are for building an old version which doesn't support LZMA. We should update them to build a newer version.