add -Wno-stringop-truncation to CFLAGS on linux

add -Wno-stringop-truncation to CFLAGS on linux

GCC>=8 supports truncation checking, however the logic is somewhat
fragile when it comes to evaluating strncpy().

In buffer.h we have implemented a wrapper called strncpynt() which
ensures we always do the right hting in the code and reduce the chance
of having bugs.

This said, it seems that the gcc logic is not able to always understand
if we are doing the right thing and throws a false positive.

Toa void the noise, disable truncation checking on Linux by default.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20191110100323.13206-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19085.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>