Add builds with \MD flag for building Python extensions

[tkralphs]

Try adding builds with the \MD flag.

[jhmgoossens]

What is the aim? To build using the Microsoft runtime as DLL rather than the Microsoft runtime as static lib?

[jhmgoossens]

There's something wrong with the MSVC binaries built with MT: missing libgcc??
Also, apologies, I'm not sure what should be the outcome of "MT" or "MD" builds. Are we expecting either statically linked COIN libaries (MT), or DLLs for the CoinUtils etc. (MD)? Or are we merely talking only about how the MS runtime libraries themselves are used?

• The PR 673 build for master:
  The MSVC MT binaries (~10MB zip) are NOT working due to missing libgcc_s_seh-1.dll and libstdc++-6.dll
  The MSVC MD binaries (~10MB zip) are working and no reference to libgcc_s_seh and libstdc++-6., and with CoinUtils-0.dll etc.

• The PR 674 build for stable:
  The MSVC MT binaries (~14.7MB zip) are NOT working due to missing libgcc_s_seh-1.dll and libstdc++-6.dll
  The MSVC MD binaries (~17.7MB zip) are working and no reference to libgcc_s_seh and libstdc++-6., but no CoinUtils-0.dll etc. Only one large executable for Clp.exe and Cbc.exe

@tkralphs I hope I'm not misunderstanding something here? Or do I have wrong expectations?
For reference, see Issue 590.