Fix for a change in CPP's behaviour in gcc 2.96 relative to 2.95.
Unlit used to inject '# <line> <file>' at the beginning of the output
file, but in gcc 2.96 this is ignored.  Instead we have to inject
'#line <line> <file>', which in turn means that GHC's lexer has to
understand this kind of pragma in addition to the plain '#' form, in
the case when we aren't running CPP after unlitting.