Prevent bitrot on configure-switched options.

[Robert Collins]

I was reading the gnu coding standards the other day. One suggestion 
therein that I quite liked is to (where system headers permit) change:

#if OPTION
 // some code here
#endif
to
 if (OPTION) {
     // some code here
  }
Their rational is that:
* The compiler then checks all code paths for parsability, scoping 
issues, variable renames ettc.
* The resulting code should be identical with any reasonable optimising 
compiler.
* It makes software with many options easier to maintain.

Thoughts?
Rob