Add WinSDK v7.0 requirement for building on Windows/x64.

doc/install.html

@@ -11,7 +11,7 @@
 <style type="text/css">
 table.compat {
   line-height: 1.2;
-  width: 35em;
+  width: 40em;
 }
 tr.compathead td {
   font-weight: bold;
@@ -84,7 +84,7 @@ operating system, CPU and compilers:
 <tr class="even">
 <td class="compatos">Windows 98/XP/Vista/7</td>
 <td class="compatcc">MSVC (EE)<br>Windows SDK<br>MinGW (GCC)<br>Cygwin (GCC)</td>
-<td class="compatcc">MSVC<br>Windows SDK</td>
+<td class="compatcc">MSVC + SDK v7.0<br>Windows SDK v7.0</td>
 </tr>
 <tr class="odd">
 <td class="compatos">OSX 10.3-10.6</td>