make sure we respect the system libffi setting in our build config.
the compiler probing is fragile and can break in some situations.

--- a/setup.py
+++ b/setup.py
@@ -2069,7 +2069,7 @@ class PyBuildExt(build_ext):
         return True
 
     def detect_ctypes(self, inc_dirs, lib_dirs):
-        self.use_system_libffi = False
+        self.use_system_libffi = ('--with-system-ffi' in sysconfig.get_config_var("CONFIG_ARGS"))
         include_dirs = []
         extra_compile_args = []
         extra_link_args = []
@@ -2113,8 +2113,6 @@ class PyBuildExt(build_ext):
             # OS X 10.5 comes with libffi.dylib; the include files are
             # in /usr/include/ffi
             inc_dirs.append('/usr/include/ffi')
-        elif '--without-system-ffi' in sysconfig.get_config_var("CONFIG_ARGS"):
-            return
 
         ffi_inc = [sysconfig.get_config_var("LIBFFI_INCLUDEDIR")]
         if not ffi_inc or ffi_inc[0] == '':
@@ -2141,10 +2141,10 @@ class PyBuildExt(build_ext):
                     ffi_lib = lib_name
                     break
 
-        if ffi_inc and ffi_lib:
+        if ffi_inc:
             ext.include_dirs.extend(ffi_inc)
+        if ffi_lib:
             ext.libraries.append(ffi_lib)
-            self.use_system_libffi = True
 
         if sysconfig.get_config_var('HAVE_LIBDL'):
             # for dlopen, see bpo-32647