diff --git a/src/FDM/JSBSim/input_output/string_utilities.h b/src/FDM/JSBSim/input_output/string_utilities.h
index d25e258b3..9bb120b04 100644
--- a/src/FDM/JSBSim/input_output/string_utilities.h
+++ b/src/FDM/JSBSim/input_output/string_utilities.h
@@ -73,10 +73,14 @@ CLASS DECLARATION
   extern std::string& to_lower(std::string& str);
   extern bool is_number(const std::string& str);
   std::vector <std::string> split(std::string str, char d);
-/* Comment out to_string functions when they are defined already - C++ 11 defines these */
+
+// libc++ has these as built-ins for all C++ language versions
+#if !defined(_LIBCPP_VERSION)
   extern std::string to_string(int);
   extern std::string to_string(double);
   extern std::string to_string(float);
+#endif
+
   extern std::string replace(std::string str, const std::string& old, const std::string& newstr);
 #else
   #include <cctype>
diff --git a/src/FDM/JSBSim/models/FGSurface.cpp b/src/FDM/JSBSim/models/FGSurface.cpp
index 70a89fcf6..88fec297e 100644
--- a/src/FDM/JSBSim/models/FGSurface.cpp
+++ b/src/FDM/JSBSim/models/FGSurface.cpp
@@ -64,7 +64,7 @@ FGSurface::FGSurface()
   isSolid = true;
 }
 
-FGSurface::FGSurface(FGFDMExec* fdmex = NULL)
+FGSurface::FGSurface(FGFDMExec* fdmex)
 {
   frictionFactor = 1.0;
   rollingFCoeff = 0.02;