Added sources and changed Makefiles to build AdbWin*.dll

The changeset adds the AdbWinApi.dll and AdbWinUsbApi.dll sources and modifies the Makefiles so that (after setting up the proper environment), these DLLs can be built from source.

Downloading WinDDK 7.1 is a pre-requisite for building these DLLs. The default settings install the DDK to C:\WinDDK\7600.16385.1. If you install it somewhere else, change the first line of _android-tools\adb-win-api\make_driver.bat_. WinDDK 7.1 can be installed here:
https://www.microsoft.com/en-us/download/details.aspx?id=11800

I wasn't sure if I should make a new target or not, so for now, the top level Makefile attempts to build AdbWin*.dll if it is configured to build adb from source and it is being executed from Windows. Let me know if this should be in a separate target or organized in a different way.