autotools: add WARN_CFLAGS to all targets

... minus test/ttmtest. The latter is not really hooked up with the
actual build.

This will give us 66 warnings on a distribution build of which
 - 12 -Wunused-variable
 - 11 -Wunused-function
 - 19 -Wmissing-prototypes
and a few -Wswitch-enum, -Wtype-limits etc.

Adding the CFLAGS gives some exposure to these so that we can fix them.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>