On modern macOS, compiler wrappers like `/usr/bin/cc` automatically
choose a SDK to pass via `-isysroot` to an underlying compiler from
Xcode or the CommandLineTools. Other toolchains like Homebrew's `gcc-*`
come with a default SDK too. Therefore, when targeting macOS, we no
longer need to choose any SDK or pass an `-isysroot` flag by default.
Update initialization of `CMAKE_OSX_SYSROOT` to be empty by default when
targeting macOS.
Fixes: #19180