Like clockwork, my matlab updates have gotten out of sync with Xcode updates. It seems like fixing this SDK error always requires a different hack each year. This year I got the error:

No supported compiler or SDK was found. For options, visit
http://www.mathworks.com/support/compilers/current_release/.


To fix it, I replaced all occurrences of 10.9 with 10.11 in /Applications/MATLAB_R2017a.app/bin/maci64/mexopts/clang{++,}_maci64.xml

ld: warning: object file was built for newer OSX version (10.11) than being linked (10.9)