#106 (C024) better ifdef and pragma support

[Wade Guthrie]

I think this is getting all the ANSI preprocessor
directives, supporting more than one level of nested
if, and allowing complex expressions in an 'if'.
Pragma support would only really be necessary if we had
an idea of a specific pragma we wanted to support (I,
personally, dislike pragmas anyway so I don't think
it'd be a biggie if there were no pragmas we wanted to
support).