Ghostscript 9.15

[Masamichi HOSODA]

I've succeed to upgrade GUB's ghostscript to 9.15 in this branch.
https://github.com/trueroad/gub/tree/ghostscript-9.15

I've succeed GUB's ``make lilypond'' by ghostscript-9.15.
All lilypond installers have been build.

In mingw (Windows):
Ghostscript can handle unicode filenames.
I've tested lilypond installer and lilypond. They work fine.
Correct PDF is generated.

In linux-64, linux-x86, linux-ppc, freebsd-64, freebsd-x86, darwin-x86:
I've tested lilypond installer and lilypond. They work fine.
Correct PDF is generated.

In darwin-ppc (PowerPC Mac):
It is untested.
I can't prepare PowerPC Mac.

Would you pull this branch for next lilypond developer build?