Use -Wall -Werror (AM_CFLAGS) when building any module.

Use -Wall -Werror (AM_CFLAGS) when building any module.

A few plugins have to set custom CFLAGS. When using "<plugin>_la_CFLAGS =",
$(AM_CFLAGS) will no longer be used when compiling <plugin> as that resets the
CFLAGS to the empty string. So, in that case, we have to explicitly add
$(AM_CFLAGS) to the custom CFLAGS of that plugin.

After doing so, a couple of problems have been identified which have been
fixed in this patch as well. That includes a possible SEGFAULT in the snmp
plugin which was missing an argument to WARNING().

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>