Introduce compatibility mode

The compiler now has a new command-line option (-Z) that toggles
between normal mode and "compatibility" mode.

When this mode is on the compiler attempts to be compatible with
the current SA-MP compiler, which allows compiling old SA-MP scripts
that would not compile otherwise and at the same time benefiting
from new features and bug fixes.

In particular, this allows compiling code that relies on the auto
generated #include guard symbols (_inc_filename).

And in case you're wondering, the Z in "-Z" does not stand for "Zeex",
it just means "a letter that is unlikely to be ever taken by another
option".

This closes #23.