Compile time check for GC?

[Ola Fosheim Grøstad]

Is there some way for library authors to test whether a GC is 
present at compile time? @nogc just means that my code does not 
depend on a GC, but it doesn't tell the compiler that my code is 
incompatible with GC.

I want to compute pointers in a way that is not scannable by a 
conservative collector for performance reasons. So I need to make 
sure that the faster code isn't chosen when people use GC using 
"static if"s.