Support C-preprocessor-style #define/#ifdef

[cxw42]

One common use case for the C preprocessor is conditional compilation using #ifdef/#else/#endif. Those can already be done in perlpp with <? if(...) { ?> <? } ?>. Similarly, #define/#undef can already be done with variable assignments. However, I think syntactic sugar for those functions, together with a -D command-line option, would make it easier for people to transition from cpp to perlpp.

I have an example implementation in branch defines of my fork. It fills %Defs for use at generation time and %D for use at execution time. Any thoughts on this approach? An alternative would be to merge %D and %S (-s name=val options). I'll leave this issue open for a bit in case anyone has any ideas or concerns.

[cxw42]

One difference between -D and -s: -D in C does textual substitution in the input file. I have updated defines to do so --- let me know what you think. I am currently thinking that I will bring -s back from master, and provide both for flexibility.