Graphite2 is failing its test suite on core-updates. [0]

https://hydra.gnu.org/build/1923151

This patch cherry-picks the upstream change that builds graphite2 with
-ffloat-store to avoid rounding errors on some platforms:

https://github.com/silnrsi/graphite/commit/f7bbdd87d510d587c872e314d6458160c0069c65

There is some more discussion in this Debian bug report:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=855814