Add SDK path on macOS

macOS removed `/usr/include` in recent versions of macOS.
This forces the usage of the internal libffi although macOS has a usable one itself.

Related to ffi#757

ext/ffi_c/extconf.rb

@@ -8,7 +8,8 @@ def system_libffi_usable?
     # We need pkg_config or ffi.h
     libffi_ok = pkg_config("libffi") ||
         have_header("ffi.h") ||
-        find_header("ffi.h", "/usr/local/include", "/usr/include/ffi")
+        find_header("ffi.h", "/usr/local/include", "/usr/include/ffi") ||
+        (find_header("ffi.h", `xcrun --sdk macosx --show-sdk-path`.strip) rescue false)
 
     # Ensure we can link to ffi_call
     libffi_ok &&= have_library("ffi", "ffi_call", [ "ffi.h" ]) ||