[m17n-list] m17n-lib will fail to build from source with new freetype 2.9

[Boyuan Yang]

Dear m17n-lib developers,

I would like to refer you to a downstream (Debian) bug report on
compatibility with upcoming freetype 2.9:

https://bugs.debian.org/892350

To be short, freetype will remove "freetype-config" tool and only
allow using pkg-config interface to detect its library. All related
code in configure.ac must be updated to prevent failing to build from
source code.

Thanks for your attention and please feel free to contact me if
there's any further questions.

--
Regards,
Boyuan Yang (Debian Developer)