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"Could not find a working Foundation implementation" has been reported to us a couple times before (#28452, #30298) but we were never able to find out why it happened.
Do you have anything in /usr/local? If so, that might be interfering; try moving it aside.
Do you have /System/Library/Frameworks/Foundation.framework? Do you have /System/Library/Frameworks/Foundation.framework/Headers/Foundation.h? If not, reinstall OS X.
Compiling MacPorts manually should of course work, but out of curiosity, why do you want to do so instead of using our pre-made disk images?
Since we are about to release MacPorts 2.1.0, you could also test whether version 2.1.0 beta 1 works better, but I don't recall us making any changes with regard to this configure check so I doubt it.