Change wxGTK build slave configuration to use built-in libpng.

This is an attempt to debug the failure of ImageTestCase::ScaleCompare() unit
test.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74975 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

build/buildbot/config/brandt32.xml

@@ -14,7 +14,7 @@
     <scheduler>trunk_quick</scheduler>
     <steps>
         <checkout/>
-        <configure options="--enable-compat28 --enable-debug"/>
+        <configure options="--enable-compat28 --enable-debug --with-libpng=builtin"/>
         <compile-all/>
         <run-tests/>
     </steps>