distutils: Discard cffi's libffi on MSVC

cffi includes a near replica of the CPython 3.7's libffi on MSVC,
which needs to be discard to avoid duplicate syms.

Author: jayvdb

pyoxidizer/src/distutils/_msvccompiler.py

@@ -586,6 +586,11 @@ def extension_link_shared_object(self,
         # directory that doesn't outlive this process.
         object_paths = []
         for i, o in enumerate(objects):
+            if 'libffi_msvc' in o:
+                print('Ignored static {}'.format(o))
+                # https://github.com/indygreg/python-build-standalone/issues/23
+                # cffi includes a near replica of CPython's custom libffi.
+                continue
             p = os.path.join(dest_path, '%s.%d.o' % (name, i))
             shutil.copyfile(o, p)
             object_paths.append(p)