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>