compilecode: Make internal _compilecode() function static.

Implement negated character classes: [^a-c].

README: Generic character classes (sets) were implemented.

regexp.h: Add double-include protection.

Make fatal() to have prefix too.

The idea is that 3rd party app may define its own re1_5_fatal(), or can
use a macro to define it to something else, e.g.
"-Dre1_5_fatal(x)=assert(!x)".