Fix cSkipStatement()'s stopping at TK_AST & co
Handle PP #if block indentation via flags on the AST nodes
pp: Fix #if evaluator

remaining problem: Doesn't handle blocks with multiple true blocks
properly, e.g.:

#if 1
#elseif 0
#else
#endif

Here the #else is preserved even though it can never be reached.
pp: Overhaul #if evaluation