Dlang equivalent of #define/#ifdef : not... version

[ichneumwn]

Hi,

Trying to convert a C header to D. The underlying package exists 
in different versions and may or may not expose certain 
functionality (modules/extensions). One should check at compile 
time that the header provides the relevant definitions, through 
#ifdef, and do a further run-time check to confirm the 
functionality is really present. It is the compile time check 
that I am finding tricky to do/emulate.

     .h : #define i_am_a_feature 1

The C-code uses this define as a guard:

     .c : #ifdef i_am_a_feature

What would the the D equivalent? This is my attempt so far:

features.d:

     import std.traits;

     private enum capabilities {
         i_am_a_feature
     }

     template supported(string member)
     {
         enum bool supported = hasMember!(capabilities, member);
     }

     version = vs_i_am_a_feature;

     enum can_i_test_for_this;

and use.d:

     import features;
     import std.stdio;

     void main() {
         static if (supported!("i_am_a_feature")) {
             writeln("Feature 1!");
         }
         static if (supported!("i_am_not_a_feature")) {
             writeln("Feature 2!");
         }
         version(vs_i_am_a_feature) {
             writeln("If only I worked");
         }
     }

This produces "Feature 1!", so the supported() path works, but is 
a bit of a round-about way and all "capability flags" need to put 
in that single enum capabilities instead of being allowed to be 
scattered across the features.d module.

As is documented, "version" does not cross the module boundary.

So my questions:
- is there a module-crossing equivalent of "version"?
- if not, is there some way I could test for the existence of the 
enum can_i_test_for_this? A SymbolExists!() or ModuleHasSymbol!() 
or ModuleHasMember!() ?

Cheers