Upgrade build script to make it compile on macosx 10.8

Author: cmaglie

build.xml

@@ -73,8 +73,8 @@ questions.
             <arg value="-arch"/>
             <arg value="x86_64"/>
             <arg value="-isysroot"/>
-            <arg value="/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk"/>
-            <arg value="-mmacosx-version-min=10.7"/>
+            <arg value="/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk"/>
+            <arg value="-mmacosx-version-min=10.8"/>
             <arg value="appbundler/native/main.m"/>
         </exec>
     </target>