configuration doesn't detect freebsd amd64 machine properly

[john.marino]

The configuration file uses the system compiler to determine the arch. For FreeBSD AMD64, the dumpmachine value is "amd64-undermydesk-freebsd".

For most BSDs, the x86_64 is identified as amd64, so amd64 should be made to be a synonym for x86_64 so that matreshka properly configures.

[vadim.godunko]

In [2130]:

Detect x86_64 as amd64.

Refs #120.