Fix compilation with gcc8

[de-vri-es]

This PR fixes compilation with GCC 8, and explicitly asks for python2 for running scripts.

Specifically:

• Turns the error for unsupported compilers into a warning.
• It removes the unused function __errno that doesn't follow C syntax.
• It fixes some out of bounds array access that are detected by GCC 8 (and -Werror makes it an error).
• Explicitly uses python2, since many of the scripts are not python3 compatible.

I've used these modifications to compile and upload firmware to two separate PX4Flow camera's, and they're working as expected.

[de-vri-es]

Thanks for the quick review and merge :)