Merge gameboy stuff and add DEF() fix

The DEF() fix fixes the following:

```
SECTION "HOME", HOME
_VARIABLE EQUS "TRUE"
IF DEF(_VARIABLE)
include "true.asm"
ENDC
```
Originally, rgbasm would check if the symbol "TRUE" was set, because "_VARIABLE" was expanded to "TRUE" before the check happened. This fixes that to check if "_VARIABLE" is set instead.