BUILD: makefile: build flags.c before haproxy to speed up the build

BUILD: makefile: build flags.c before haproxy to speed up the build

The end of the build is often super slow. In practice it's flags.o that
now takes ages (3.4 seconds) and blocks everything on a single core at
the end. Let's declare it before the haproxy target so that it starts
earlier. On a quad-2.2 GHz CPU, the build time goes down from 44 to 42s
and the end feels less painful.