Add support for `undef

[gjcoram]

[root@5033e3bcb6f9 io]# target/release/openvaf r3_cmc.va error: macro 'undef' has not been declared --> /io/r3_cmc.va:127:9 | 127 | undef VAMS_COMPACT_MODELING | ^^^^^^ macro not found here

error: unexpected token identifier; expected 'discipline', 'nature' or 'module' --> /io/r3_cmc.va:127:16 | 127 | `undef VAMS_COMPACT_MODELING | ^^^^^^^^^^^^^^^^^^^^^^^^^ expected 'discipline', 'nature' or 'module'

error: could not compile r3_cmc.va due to 2 previous errors