Request assistance converting C's #ifndef to D

[Andrew Edwards via Digitalmars-d-learn]

The following preprocessor directives are frequently encountered in C 
code, providing a default constant value where the user of the code has 
not specified one:

	#ifndef MIN
	#define MIN     99
	#endif

	#ifndef MAX
	#define MAX     999
	#endif

I'm at a loss at how to properly convert it to D. I've tried the following:

	enum MIN = 0;
	static if(MIN <= 0)
	{
		MIN = 99;
	}

it works as long as the static if is enclosed in a static this(), 
otherwise the compiler complains:

	mo.d(493): Error: no identifier for declarator MIN
	mo.d(493): Error: declaration expected, not '='

This however, does not feel like the right way to do thinks but I cannot 
find any documentation that provides an alternative. Is there a better 
way to do this?

Thanks,
Andrew