Error out of configure script if submodules have not been checked out

configure

@@ -1,2 +1,8 @@
 #!/bin/sh
+if [ ! -e libs/glcxx/src ]; then
+  echo Warning: It appears that you have not checked out the submodules.
+  echo Please run \'git submodule update --init\' first.
+  exit 1
+fi
+
 exec ./waf configure "$@"