It happens to me every time Xcode has new version. clang and bunch of other apps can't find old name SDK.
then i do something stupid:
cd /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
sudo ln -s MacOSX.sdk <<PUT HERE ANY SDK NAME WANTS and MISSES>>
but, how can i convince the system to do it by itself?