diff --git a/fdlibm.h b/fdlibm.h
--- a/fdlibm.h
+++ b/fdlibm.h
@@ -9,20 +9,23 @@
  * ====================================================
  */
 
 /*
  * from: @(#)fdlibm.h 5.1 93/09/24
  * $FreeBSD$
  */
 
-#ifndef _MATH_H_
-#define	_MATH_H_
+#ifndef mozilla_imported_fdlibm_h
+#define	mozilla_imported_fdlibm_h
 
+#ifndef M_PI_2
 #define	M_PI_2		1.57079632679489661923	/* pi/2 */
+#endif
+
 double	acos(double);
 double	asin(double);
 double	atan(double);
 double	atan2(double, double);
 double	cos(double);
 double	sin(double);
 double	tan(double);
 
@@ -54,9 +57,9 @@ float	tanf(float);
 float	exp2f(float);
 float	expf(float);
 float	log10f(float);
 float	logf(float);
 float	powf(float, float);
 
 float	hypotf(float, float);
 
-#endif /* !_MATH_H_ */
+#endif /* !mozilla_imported_fdlibm_h */