[core] Use std::to_string on Windows

Clang on Window's can't parse rapidjson's dtoa.

src/mbgl/util/dtoa.cpp

@@ -1,10 +1,16 @@
 #include "dtoa.hpp"
 
+// Clang/C2 on Windows 64-bits can't parse rapidjson's dtoa
+// and it was causing the compiler to crash.
+#if !defined(_WINDOWS)
 #include <rapidjson/internal/dtoa.h>
+#endif
 
 namespace mbgl {
 namespace util {
 
+#if !defined(_WINDOWS)
+
 namespace {
 
 // From https://github.com/miloyip/rapidjson/blob/master/include/rapidjson/internal/dtoa.h
@@ -101,5 +107,13 @@ std::string dtoa(double value) {
     return data;
 }
 
+#else
+
+std::string dtoa(double value) {
+    return std::to_string(value);
+}
+
+#endif
+
 } // namespace util
 } // namespace mbgl

src/mbgl/util/dtoa.hpp

@@ -5,7 +5,6 @@
 namespace mbgl {
 namespace util {
 
-char* dtoa(double value, char* buffer);
 std::string dtoa(double value);
 
 } // end namespace util