Build documentation (building.md) needs updating as a result of 8170681

[Michael D]

Hi All,

Couldn't find a documentation mailing list so I figured I'd drop this
here since it seems most relevant to the build system.
As a result of last weeks change for 8170681 - removal of fontconfig
from the source tree, fontconfig was introduced as a build depend
(libfontconfig1-dev on Debian).
As such doc/building.md should be updated to include some of this info.
I've prepared a cookie cutter patch here
https://patch-diff.githubusercontent.com/raw/md-5/OpenJDK/pull/2.patch
but you are more than welcome to DIY.

Thanks
Michael