This commit adds default search paths for system frameworks
on macOS while also adding `-isysroot` for OS versions at least BigSur.
Since BigSur (11.0.1), neither headers nor libs exist in standard
root locations (`/usr/include`, `/System/Library/Frameworks`). Instead, they
are now exclusively part of the installed developer toolchain (either
via XCode.app or CLT), and specifying `-isysroot` allows us to keep
using universal search paths such as `/System/Library/Frameworks` while
only changing the include flag from `-iframework` to
`-iframeworkwithsysroot`.