ROOT (or rather its interpreter cling) checks at startup where to find the C++ headers. For that it invokes the compiler, piping in #include <new> and checking its -v output which mentions the include paths.
For you, for some reason, that hangs: cling never returns from parsing / waiting for the output. We need to figure out why and what happens.
Additionally, and maybe connected, ROOT (or rather cling) needs Xcode, not just the command line tools. So seeing this output above makes me wonder whether these issues are connected.
This seems to be your own build of ROOT, correct? If so, edit the file interpreter/cling/lib/Interpreter/CIFactory.cpp and replace
if (Verbose)
cling::log() << "Looking for C++ headers with:\n " << CppInclQuery << "\n";
with
//if (Verbose)
cling::log() << "Looking for C++ headers with:\n " << CppInclQuery << "\n";
Then build (just the target Cling is good enough, so make Cling or ninja Cling), start ROOT. It’s now telling you the exact line it invokes and that hangs. Run that from your terminal - does that work?