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);