Missing support for underscores in identifier names

[marcauberer]

I wonder why we did not notice this earlier, but CCom currently does not support underscores in identifiers, which is definitely not the intended solution.
The problem is that isalnum() does not return true when the input is an underscore.

[marcauberer]

@Splines As far I have seen, the Java compiler is also affected by this issue.

[Splines]

thanks for noticing, will fix this