When gcc 7 is used, -Werror=format-truncation finds snprintf() calls in
mod_cdr_mongodb.c and suspects that they may result in truncation. As a
result the build fails.
Work around this by adding -Wno-error=format-truncation to CFLAGS when
gcc support this.
JIRA-10939 has been raised upstream to follow up on this.
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>