Autotools windows

- windows builds with autotools (./configure style)
- tested under cygwin (32bit), msys and mingw

AM_CONDITIONAL(HOST_IS_WINDOWS, ...

assumes you will be using it with AdbWinApi as .DLL files, however it does not depend on it during building:

- added .DEF files and rules in Makefile.am which generate import libs for AdbWinApi.dll & AdbWinUsbApi.dll
- added corresponding adb_api.h to src/