diff --git a/svgalib/threeDKit/quickmath.h b/svgalib/threeDKit/quickmath.h

index c14b1b2..2928564 100644

--- a/svgalib/threeDKit/quickmath.h

+++ b/svgalib/threeDKit/quickmath.h

@@ -60,7 +60,7 @@ typedef struct {

 #define max(x,y)     (((x) > (y)) ? (x) : (y))

 #define min(x,y)     (((x) < (y)) ? (x) : (y))

-#ifndef __GNUC__

+#ifndef _GNU_SOURCE

 double fsqr (double x);

 int lsqr (int x);