[Fontconfig] Detecting whether a font is a fallback

[Werner LEMBERG]

> If we were starting the LilyPond project today, that might make
> sense.  However, there are plenty of existing LilyPond files that
> use font family names like “times new roman” that are substituted
> with “Times New Roman”, and I think we'd rather not start making
> those emit warnings.

I disagree.  We already emit a lot of 'please fix manually' warnings
in `convert-ly` (for non-LilyPonders: this script updates the input
syntax of LilyPond files from older to newer versions, but sometimes
it can't do that automatically, thus this warning); I don't see any
reason why urging users to fix incorrectly specified family names
should be an exception – a warning in LilyPond is definitely justified
IMHO.


    Werner