Make the lexer return SPACE tokens unconditionally.

Make the lexer return SPACE tokens unconditionally.

It seems strange to always be returning SPACE tokens, but since we
were already needing to return a SPACE token in some cases, this
actually simplifies our lexer.

This also allows us to fix two whitespace-handling differences
compared to "gcc -E" so that now the recent modification to the test
suite passes once again.