$ sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer
$ sudo port -v upgrade installed
[...]
checking for gcc... /Applications/Xcode.app/Contents/Developer/usr/bin/clang
checking for C compiler default output file name...
configure: error: C compiler cannot create executables
See `config.log' for more details.
[...]
$ xcrun -find clang
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang

MEGAFAIL! :(