Make = optional in Global directive

[erkyrath]

The compiler supports:

Constant c1 1;
Constant c2 = 2;
Global g1 = 1;

But not:

Global g2 2;

This has been tripping people up for a long time.

The original reason for the mandatory = was to distinguish it from this case:

Global arr --> 4;

...but this has long since been deprecated in favor of the Array directive. (It prints an "Obsolete usage" warning.)

I think we can get rid of the Global arr --> ... syntax entirely and support Global glob val; instead.