Alpha 3.1 Windows shared libraries require MSVCR120D.DLL; OS X static libs require El Capitan

It seems that the Windows shared libraries in the **[Alpha 3.1](https://github.com/andlabs/libui/releases/tag/alpha3.1)** release were compiled in _debug_ mode, as they have a dependency on `MSVCR120D.DLL`. AFAIK, the previous **[Alpha 3](https://github.com/andlabs/libui/releases/tag/alpha3)** release was built in _release_ mode and does not include this dependency. I've attached screenshots of all four libraries from Dependency Walker.

| Alpha 3 32-bit | Alpha 3 64-bit |
| --- | --- |
|  |  |

| Alpha 3.1 32-bit | Alpha 3.1 64-bit |
| --- | --- |
|  |  |