setup: avoid linker warnings on Windows about multiple export specifications

The PyMODINIT_FUNC macro contains __declspec(dllexport), and then the build
process adds an "/EXPORT func" to the command line.  The 64-bit linker flags
this [1].

Everything except zstd.c and bser.c are covered by redefining the macro in
util.h [2].  These modules aren't built with util.h in the #include path, so the
redefining hack would have to be open coded two more times.

After seeing that extra_linker_flags didn't work, I couldn't find anything
authoritative indicating why, though I did see an offhand comment on SO that
CFLAGS is also ignored on Windows.  I also don't fully understand the
interaction between msvccompiler and msvc9compiler- I first subclassed the
latter, but it isn't used when building with VS2008.

I know the camelcase naming isn't the standard, but the HackedMingw32CCompiler
class above it was introduced 5 years ago (and I think the current style was
in place by then), so I assume that there's some reason for it.

[1] https://support.microsoft.com/en-us/help/835326/you-receive-an-lnk4197-error-in-the-64-bit-version-of-the-visual-c-compiler
[2] https://bugs.python.org/issue9709#msg120859

setup.py

@@ -704,6 +704,23 b' except ImportError:'
     class HackedMingw32CCompiler(object):
         pass
 
+if os.name == 'nt':
+    # Allow compiler/linker flags to be added to Visual Studio builds.  Passing
+    # extra_link_args to distutils.extensions.Extension() doesn't have any
+    # effect.
+    from distutils import msvccompiler
+
+    compiler = msvccompiler.MSVCCompiler
+
+    class HackedMSVCCompiler(msvccompiler.MSVCCompiler):
+        def initialize(self):
+            compiler.initialize(self)
+            # "warning LNK4197: export 'func' specified multiple times"
+            self.ldflags_shared.append('/ignore:4197')
+            self.ldflags_shared_debug.append('/ignore:4197')
+
+    msvccompiler.MSVCCompiler = HackedMSVCCompiler
+
 packagedata = {'mercurial': ['locale/*/LC_MESSAGES/hg.mo',
                              'help/*.txt',
                              'help/internals/*.txt',