Try to fix some buildbot warnings

Most interesting are:
 warning: cannot optimize loop, the loop counter may overflow [-Wunsafe-loop-optimizations]
 warning: ISO C forbids zero-size array [-Wpedantic]
 warning: ISO C90 doesn't support unnamed structs/unions [-Wpedantic]
 warning: cast discards '__attribute__((const))' qualifier from pointer target type [-Wcast-qual
 warning: initializer element is not computable at load time [enabled by default]

Change-Id: I5573c6bdca856a304877d9bef643f8c0fa93cdaf
Reviewed-on: https://code.wireshark.org/review/3174
Petri-Dish: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>
extcap: Always create extcap/ dir in run/

Change-Id: I5d0a2591b9086a138cefb0019f710ffb34a34c52
Reviewed-on: https://code.wireshark.org/review/3876
Petri-Dish: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>