Bump the default _WIN32_WINNT to 0x0601, i.e. Windows 7

Since the C++ libraries require Windows 7, there's little point in
keeping the default target for C at Vista.

build-mingw-w64.sh

@@ -2,7 +2,7 @@
 
 set -e
 
-DEFAULT_WIN32_WINNT=0x600
+DEFAULT_WIN32_WINNT=0x601
 DEFAULT_MSVCRT=ucrt
 unset HOST
 
@@ -27,7 +27,7 @@ while [ $# -gt 0 ]; do
     shift
 done
 if [ -z "$PREFIX" ]; then
-    echo $0 [--skip-include-triplet-prefix] [--with-default-win32-winnt=0x600] [--with-default-msvcrt=ucrt] dest
+    echo $0 [--skip-include-triplet-prefix] [--with-default-win32-winnt=0x601] [--with-default-msvcrt=ucrt] dest
     exit 1
 fi