mempool: Require C99 flexible arrays
dbus 1.15.x officially requires C99, so we can do this unconditionally on the 1.15.x branch.
Followup for !343 (merged), not intended for backport to 1.14.x.
/cc @arichardson @thiago
dbus 1.15.x officially requires C99, so we can do this unconditionally on the 1.15.x branch.
Followup for !343 (merged), not intended for backport to 1.14.x.
/cc @arichardson @thiago