Allow overriding C/C++ flags from CMake command line

Append user's flags to our flags instead of prepending them to make
user flags higher priority.

Change-Id: I3ead33ee5fa06a2395d9197a027306ef99a704a2
Reviewed-on: https://code.wireshark.org/review/2722
Reviewed-by: Jörg Mayer <jmayer@loplof.de>
(cherry picked from commit 65e3f5c25ee48e5b736ddbdf04fb2a31601ce8c1)
Reviewed-on: https://code.wireshark.org/review/2786
Reviewed-by: Balint Reczey <balint@balintreczey.hu>