Use 'uname -m' instead of 'uname -p' to identify architecture

The latter returns "unknown" in Debian Wheezy, and that made us build
32-bit versions.  In a bare minimum 64-bit system, this leads to weird
compilation errors, because the system include headers are "broken"
(due, I'm guessing, to a unsatisfied dependency on libc6-dev-i386.)