Require OSX 10.6, 64-bit

Drop support for 32-bit or pre-10.6 Macs.

Also, some of the OSX build docs hadn't been updated since the
autotools change.

share/qt/Info.plist.in

@@ -2,6 +2,14 @@
 <!DOCTYPE plist SYSTEM "file://localhost/System/Library/DTDs/PropertyList.dtd">
 <plist version="0.9">
 <dict>
+  <key>LSMinimumSystemVersion</key>
+  <string>10.6.0</string>
+
+  <key>LSArchitecturePriority</key>
+  <array>
+    <string>x86_64</string>
+  </array>
+
   <key>CFBundleIconFile</key>
   <string>bitcoin.icns</string>