w32: Hacks for building with 32 bit mingw64.

* configure.ac (INSERT__TYPEDEFS_FOR_GPGME_H): Add hacks for 32 bit
mingw64.
* src/util.h [W32]: Include winsock2.h before windows to make mingw64
happy.
* src/w32-util.c (_WIN32_IE): Need to use 5.1 for mingw64.

It is not clear to me what mingw64 wants to achieve with their POSIX
hacks. In particular the off64_t stuff looks quite strange given that
Windows has a pretty stable API for close to 2 decades. Thus I can't
say whether it will really work when build with that toolchain.