Fix some regressions for CentOS in the 1.20 branch
cerbero: Fix python 3.6 compat (regression)
The heartbeat is purely cosmetic anyway.
Fixes #400 (closed)
fontconfig.recipe: Explicitly use gnu99 as the C std
This is required when using older toolchains, such as CentOS, and is
a regression due to a backported version bump.
Fixes #401 (closed)