Issue 5000/1: Add checking Ghostscript 9.20+ and Extract PDFmark

This commit adds checking Ghostscript 9.20+ and Extract PDFmark.
They can reduce the disk space required for document building.
They also can reduce the size of generated PDF document files.

Even if they are missing, you can build documents.

Ghostscript 9.20+:
It is necessary for string named destinations.
Ghostscript 9.19 and prior cannot handle them.
http://bugs.ghostscript.com/show_bug.cgi?id=696974

Extract PDFmark:
It is necessary for preserving named destinations and page mode.
https://github.com/trueroad/extractpdfmark
http://www.ctan.org/pkg/extractpdfmark