Fix Bessel

Author: Lukas Korous

hermes2d/test_examples/05-hcurl-adapt/bessel.cpp

@@ -1875,7 +1875,7 @@ extern double sin ( double );
 // extern double p1evl ( double, void *, int );
 // extern double floor ( double );
 extern double fabs ( double );
-extern int isnan ( double );
+extern bool isnan ( double );
 // extern int isfinite ( double );
 static double stirf ( double );
 double lgam ( double );