In trying to track down a build failure of clang-3.7 on El Cap, I found it only builds there if “use_xode yes” is selected...
In trying to understand why that is, I find this odd SDK difference. It’s on two different El Cap machines I have. Does everyone with El Cap have this issue - the last Xcode has MacOSX10.11 SDK but the CLTs have MacOSX10.12 SDK? If so, no wonder we see strange errors: $ ls -la /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs total 0 drwxr-xr-x 3 root wheel 102 3 Mar 2016 . drwxr-xr-x 5 root wheel 170 26 Apr 2016 .. drwxr-xr-x 5 root wheel 170 20 Oct 2020 MacOSX10.11.sdk $ ls -la /Library/Developer/CommandLineTools/SDK* total 8 drwxr-xr-x 4 root wheel 136 1 Jul 2017 . drwxr-xr-x 5 root admin 170 1 Jul 2017 .. drwxr-xr-x 5 root wheel 170 17 Sep 2017 MacOSX.sdk lrwxr-xr-x 1 root wheel 10 1 Jul 2017 MacOSX10.12.sdk -> MacOSX.sdk