arch checking on depends_build

[Jeremy Huddleston]

We can handle packages that have not architecture (xorg protos and other "data" ports), but there doesn't seem to be a way to work around needing an executable (don't care about arch) as a build dependency...  Eg:

$ sudo port -v install xulrunner-devel +internal_dependencies
Error: Cannot install xulrunner-devel for the arch(s) 'i386' because
Error: its dependency autoconf213 is only installed for the arch 'x86_64'
Error: and does not have a universal variant.
Error: Unable to execute port: architecture mismatch
To report a bug, see <http://guide.macports.org/#project.tickets>

$ sudo port -v install xorg-server
Error: Cannot install xorg-server for the arch(s) 'i386 x86_64' because
Error: its dependency doxygen is only installed for the arch 'x86_64'
Error: and does not have a universal variant.
Error: Unable to execute port: architecture mismatch
To report a bug, see <http://guide.macports.org/#project.tickets>