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

gdb/configure

@@ -24969,7 +24969,17 @@ if ${gdb_cv_cxx_std_thread+:} false; then :
 else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-#include <thread>
+  #if defined (__MINGW32__) || defined (__CYGWIN__)
+    # ifdef _WIN32_WINNT
+    #  if _WIN32_WINNT < 0x0501
+    #   undef _WIN32_WINNT
+    #   define _WIN32_WINNT 0x0501
+    #  endif
+    # else
+    #  define _WIN32_WINNT 0x0501
+    # endif
+    #endif	/* __MINGW32__ || __CYGWIN__ */
+    #include <thread>
     void callback() { }
 int
 main ()