The gcc 8 compiler introduces a warning for certain string
manipulation functions, flagging usages which _may_ not be intended.
An audit of the iPXE sources indicates all usages of strncat and
strncpy are as intended, so the warnings currently issued are not
helpful, especially if warnings are considered errors.

Fix by detecting gcc's support for -Wno-stringop-truncation and, if
detected, using that option to avoid the warning.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Also-fixed-by: Christian Hesse <list@eworm.de>
Also-fixed-by: Roman Kagan <rkagan@virtuozzo.com>
Also-fixed-by: Bernhard M. Wiedemann <bwiedemann@suse.de>
Also-fixed-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>