Fixed compilation error of irtkMatrix.h on MAc regarding friend declarations

Author: Stefan Pszczolkowski

geometry++/include/irtkMatrix.h

@@ -171,9 +171,9 @@ class irtkMatrix : public irtkObject
   // Matrix square root.
   friend irtkMatrix sqrtm(irtkMatrix);
 
-  friend irtkMatrix FrechetMean(irtkMatrix *, int, int = 10);
+  friend irtkMatrix FrechetMean(irtkMatrix *, int, int);
 
-  friend irtkMatrix FrechetMean(irtkMatrix *, double *, int, int = 10);
+  friend irtkMatrix FrechetMean(irtkMatrix *, double *, int, int);
 
 #ifndef USE_STL
   /// Comparison operator != (if USE_STL is defined, negate == operator)
@@ -274,6 +274,10 @@ class irtkMatrix : public irtkObject
 #endif
 };
 
+//Declare FrechetMean friend functions with default value
+irtkMatrix FrechetMean(irtkMatrix *, int, int = 10);
+irtkMatrix FrechetMean(irtkMatrix *, double *, int, int = 10);
+
 //
 // Access operators
 //