System libraries are no longer in the file system on macOS Big Sur

[carlocab]

This snippet checks for the existence of /usr/lib/libffi.dylib on Darwin and uses it if it is found:

nu/Nukefile

Lines 63 to 68 in 1b0cd7f

	(then (if (NSFileManager fileExistsNamed:"/usr/lib/libffi.dylib")
	(then ;; Use the libffi that ships with OS X.
	(@inc_dirs addObject:"/usr/include/ffi"))
	(else ;; Use the libffi that is distributed with Nu.
	(@inc_dirs addObject:"./libffi/include")
	(@lib_dirs addObject:"./libffi")))))

This check always fails on macOS Big Sur because system libraries can no longer be found on the file system. (See, for example, https://developer.apple.com/forums/thread/655588.)

The headers are also no longer in /usr/include -- they can be found in the SDK, if installed. (Try xcrun -show-sdk-path.)

Fixing this would also resolve #97, because the system libffi does provide ffi_prep_closure.