Revert "build-sys: avoid warnings from assert_cc"

This reverts commit f1a1264d13b31b9f5521f482d9a5a9d78da55efb.

We can turn this off with a pragma only on old gcc. Newer gcc doesn't
need this, so let's not turn this off for everybody.
bus: when turning a buffer into a sd_bus_message avoid creating zero-length parts