Having a similar issue compiling ROOT on a OSX Catalina machine. CLI tools are installed and looks like cling is looking for the headers in the wrong place, namely
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/
instead of
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
Build log attached
rootbuild.txt (1.0 MB)
Cheers,
Valerio