Add target to quickly compile a snapshot version
Correct error message when no fontset was found
In use_mb mode, use a more reasonable default/fallback font.

'fixed' does not exist for the multibyte font searching methods, and it does
not make sense to have this be a compile-time option.
Be more forgiving when XCreateFontSet creates a fontset with missing charsets

Especially utf-8 locales list unreasonable amounts of charsets as requirements.
When a font can be found that satisfies some, but not all of them, we just use
the font and write a warning into the logging. A user who really cares about
all those charsets can fix his configuration himself.
Compile warning