Target flags sometimes tested with #if other times with #ifdef.

[hoodmane]

Pretty much all of the guards for FFI_CLOSURES and FFI_NATIVE_RAW_API test with #if so it's possible to explicitly say "We don't support this" with #define FFI_CLOSURES 0. On the other hand, the tests for FFI_GO_CLOSURES mostly test with #ifdef so #define FFI_GO_CLOSURES 0 is treated as a signal that they are supported. Same with FFI_TARGET_SPECIFIC_VARIADIC. It would be nice if this were consistent. I think using #if is better so targets can explicitly declare that they don't support things.