Unselect universal variant if there ends up being only one arch

[ryandesign (Ryan Carsten Schmidt)]

If I am on Snow Leopard (default build_arch is x86_64) and have a port that limits building to 32-bit ("supported_archs i386 ppc") and I request the universal variant, the port will end up installed for only i386. "port -v installed" shows the port is installed with only i386 arch, but it still shows the universal variant is selected.

$ port -v installed twain-sane
The following ports are currently installed:
  twain-sane @2.5_1+universal (active) platform='darwin 10' archs='i386'

Suggestion: If the intersection of universal_archs and supported_archs leaves only a single arch, MacPorts should unselect the universal variant.

[ryandesign (Ryan Carsten Schmidt)]

A different solution was implemented for MacPorts 2.0.0: An error message is now printed:

Error: twain-sane cannot be installed for the configured universal_archs 'x86_64 i386' because it only supports the arch(s) 'i386 ppc'.