Make WIN32_WINNT defines conditional

Requested by Gisle Vanem on tor-dev.  I'm not quite sure this is the
right solution, but it's probably harmless.

changes/ifdef-winnt

@@ -0,0 +1,4 @@
+  o Build changes:
+    - On Windows, we now define the _WIN32_WINNT macros only if they
+      are not already defined. This lets the person building Tor decide,
+      if they want, to require a later version of Windows.

src/common/compat.h

@@ -9,8 +9,12 @@
 #include "orconfig.h"
 #include "torint.h"
 #ifdef MS_WINDOWS
+#ifndef WIN32_WINNT
 #define WIN32_WINNT 0x400
+#endif
+#ifndef _WIN32_WINNT
 #define _WIN32_WINNT 0x400
+#endif
 #define WIN32_LEAN_AND_MEAN
 #if defined(_MSC_VER) && (_MSC_VER < 1300)
 #include <winsock.h>

src/common/crypto.c

@@ -13,8 +13,12 @@
 #include "orconfig.h"
 
 #ifdef MS_WINDOWS
+#ifndef WIN32_WINNT
 #define WIN32_WINNT 0x400
+#endif
+#ifndef _WIN32_WINNT
 #define _WIN32_WINNT 0x400
+#endif
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #include <wincrypt.h>

src/common/tortls.c

@@ -22,8 +22,12 @@
 
 #include <assert.h>
 #ifdef MS_WINDOWS /*wrkard for dtls1.h >= 0.9.8m of "#include <winsock.h>"*/
+ #ifndef WIN32_WINNT
  #define WIN32_WINNT 0x400
+ #endif
+ #ifndef _WIN32_WINNT
  #define _WIN32_WINNT 0x400
+ #endif
  #define WIN32_LEAN_AND_MEAN
  #if defined(_MSC_VER) && (_MSC_VER < 1300)
     #include <winsock.h>

src/or/or.h

@@ -23,8 +23,12 @@
 #endif
 
 #ifdef MS_WINDOWS
+#ifndef WIN32_WINNT
 #define WIN32_WINNT 0x400
+#endif
+#ifndef _WIN32_WINNT
 #define _WIN32_WINNT 0x400
+#endif
 #define WIN32_LEAN_AND_MEAN
 #endif