Set _WIN32_WINNT in common.m4 configure check

GCC recently added support for the Windows thread model, enabling
libstdc++ to support Windows natively.  However, this supporrt
requires a version of Windows later than the minimum version that is
supported by GDB.

PR build/29966 points out that the GDB configure test for std::thread
does not work in this situation, because _WIN32_WINNT is not defined
in test program, and so <thread> seems to be fine.

This patch is an attempt to fix the problem, by using the same setting
for _WIN32_WINNT at configure time as is used at build time.

I don't have access to one of the older systems so I don't think I can
truly test this.  I did do a mingw cross build, though.  I'm going to
ask the bug reporter to test it.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29966