Generate multiple variants for conditional compilation

[edsko]

Suppose we have a C header such as

#ifdef EXTENDED
#   define MAX_LEN 75
#else
#   define MAX_LEN 50
#endif

we have two ways to proceed:

• We could resolve the condition when we generate the headers: the value of EXTENDED would have to be known when we generate the headers. This is far by the simpler option, and should probably be the default
• We could delay the decision, and generate Haskell code that relies on the same condition:

maxLen :: forall a. fromCIntegerConstant a => a
#ifdef EXTENDED
maxLen = fromCIntegerConstant 75
#else
maxLen = fromCIntegerConstant 50
#endif

[edsko]

If we want to support this at all, we probably only want to support it in a very minimal fashion, because libclang itself does not support this mode; so if for example we wanted to query libclang for the offsets of the fields inside structs, if there is CPP involved, we'd have to run libclang for each possible CPP permutation. Marking this as a low-priority enhancement for now.