Compilation on rpi / arm about uvgrtp

[jrsnen]

Hi @Consti10, I added a CMake option to remove the -Werror flag called DISABLE_WERROR, so you or anyone else doesn't have to change the file for this. Also added a mention to the build instructions.

- Joni

[jrsnen]

I tried to fix the warning shown with 792b97d.

I'm a little hesitant to remove -Werror now that we have gotten uvgRTP to be warning free, since it is often easier to have zero tolerance for warnings than to remove them occasionally. Nevertheless, I would be ok removing it on some platforms/compilers though if there was a reason and a simple method.

[Consti10]

I totally understand. Don't also think there is immediate action required, since you can always remove the flag easily if your (esoteric ? ;) )compiler for some reason has issues.
Nonetheless, I recommend this talk: somewhere in there (can't find the timestamp anymore) there is a good explanation why Werror often is not what you want ;)