Add build support to generate PNG file from .dot file

The proposal is to add a configure option to generate .png files from .dot files. This should be enabled when generating the docs bundle for Oracle build.

The input file is $BUILD_OUTPUTDIR/images/spec-dotfiles/$MODULE.dot

The output file for the docs bundle is:
    $BUILD_OUTPUTDIR/images/docs/api/$MODULE-graph.png

This is the path where @moduleGraph taglet will depend on.

$BUILD_OUTPUTDIR/spec-dotfiles/*.dot files are generated during the `images` target using GenGraphs build tool. That can be refactored if desired but this can only be done after the exploded image is built.

This is not required by JDK-8173303 but a nice-to-have feature. When the build is configured to run `dot` to generate .png files, it would be nice to provide a make target to generate .png files in $BUILD_OUTPUTDIR/images/gengraphs and spec-dotfiles (dot -Tpng -o $MODULE.png $MODULE.dot). JDK developer can run that target when we want to look at the graph rather than invoking `dot` ourselves. I mention this because that may influence the decision in the build logic whether the build will generate .png files and do the copying to `docs/api` directory.