Add lilypond-book can use XeTeX backend for auto-detect

Add lilypond-book can use XeTeX backend for auto-detect

lilypond-book uses TeX engine for auto-detect default settings.
In the case of using XeTeX , the detection did not work.
When there is no glyphs, pdfTeX creates PDF.
However, XeTeX does not create PDF.
Threfore, the detection is failed.

[trueroad, 2016-01-19 14:37:38]

I've pushed to staging.

Issue 4734: Add lilypond-book can use XeTeX backend for auto-detect staging

author    Masamichi Hosoda <trueroad@trueroad.jp>
          Sat, 9 Jan 2016 17:13:22 +0000 (02:13 +0900)
committer Masamichi Hosoda <trueroad@trueroad.jp>
          Tue, 19 Jan 2016 13:56:59 +0000 (22:56 +0900)
commit    57000058f2521712364faa03b344cc49b193ca75