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
@@ -2052,7 +2052,7 @@
                     ffi_lib = lib_name
                     break
 
-        if ffi_inc and ffi_lib:
+        if ffi_inc:
             ffi_headers = glob(os.path.join(ffi_inc, '*.h'))
             if grep_headers_for('ffi_prep_cif_var', ffi_headers):
                 ext.extra_compile_args.append("-DHAVE_FFI_PREP_CIF_VAR=1")
@@ -2062,7 +2062,8 @@
                 ext.extra_compile_args.append("-DHAVE_FFI_CLOSURE_ALLOC=1")
 
             ext.include_dirs.append(ffi_inc)
-            ext.libraries.append(ffi_lib)
+            if ffi_lib:
+                ext.libraries.append(ffi_lib)
             self.use_system_libffi = True
 
         if sysconfig.get_config_var('HAVE_LIBDL'):