I've ran into this issue before and worked around it by putting this in a D file:

module util;
// for C files built with import C that try to use GCC/clang builtin that importC does not provide

extern(C) int __builtin_clz(int v)    
{    
    return clz(v);    
}    
    
// count leading zeros.    
int clz(uint v) pure nothrow @nogc @safe    
in(v != 0)    
{    
    import core.bitop : bsr;    
    return 31 - bsr(v);    
}


and putting
__import util
at the top of the C file