https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87030
--- Comment #11 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> ---
(In reply to Jeremy Huddleston Sequoia from comment #10)
> Given those, gcc only builds if we have the DevSDK ("headers at /" package)
> installed.
I may be misunderstanding what you say: GCC builds and runs fine without the
headers in /usr/include. At Homebrew, we are not recommending users to install
the /usr/include headers package, and we build and run GCC fine. The
configuration is the following
(https://github.com/Homebrew/homebrew-core/blob/master/Formula/gcc.rb):
--with-native-system-header-dir=/usr/include
--with-sysroot=/path/to/sdk
if the system headers are in /path/to/sdk/usr/include. Thus, on a Mojave
installation with Xcode CLT installed, we set /path/to/sdk to
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
