This message means that Berkeley doesn't know what Berkeley is saying. In short: SAH doesn't know anything about x86_64-pc-linux-gnu, which is official platform designation, but rather supports x86_64-unknown-linux-gnu which is still de-facto platfrom name used by GNU tools for software development (autoconf and friends).

You can get over this hurdle by adopting anonymous platform mechanism, which also allows you to use Simon's optimised project executables.

In future there will be another possibility: a mechanism of alternative platforms was implemented so that if project doesn't support primary (hosts native) platform, it can use alternative one (in your case this would be i386-pc-linux-gnu). This approach requires support from both client and server. Any beta client (5.10.x) supports this, while project servers are a bit slow at adopting new software versions.