https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90834
--- Comment #13 from Iain Sandoe <iains at gcc dot gnu.org> --- The /usr/local/include thing needs to be handled carefully "bare" clang (at least up to 9.x) behaves as per GCC, and prepends a given --sysroot= to the usr/local paths too. This is correct; imagine that you want to do a cross-compile to a different Darwin version, so you do ... ... gcc --sysroot=/path/to/darwinXX/SDK -mmacosx-version-min=XX foo.c ... you *do not* want that compile line to look in /usr/local/include - since that directory might have nothing to do with darwinXX (imagine different APIs). So, if we wanted to do something "automatic" it would have to be predicated on: 1) That there was no --with-sysroot= given at configure time 2) That this is not a cross-compiler 3) That there is no --sysroot= on the invocation line 4) That target = host at invocation time ==== While one could do something at configure time to choose between "/" and "Library/Developer/CommandLineTools/SDKs/MacOSX.sdk" based on build=host=target and what the build system has installed, that's guaranteed to fail if the built compiler is moved to a system with different configuration. So, we keep coming back to needing something at invocation time - and that something needs to be efficient (i.e. not a process launch)
