add user agent header

fixes #628

include/mbgl/util/constants.hpp

@@ -0,0 +1,32 @@
+#ifndef MBGL_UTIL_CONSTANTS
+#define MBGL_UTIL_CONSTANTS
+
+#include <cmath>
+
+namespace mbgl {
+
+namespace util {
+
+extern const float tileSize;
+extern const char *userAgent;
+
+}
+
+namespace debug {
+
+extern const bool tileParseWarnings;
+extern const bool styleParseWarnings;
+extern const bool spriteWarnings;
+extern const bool renderWarnings;
+extern const bool renderTree;
+extern const bool labelTextMissingWarning;
+extern const bool missingFontStackWarning;
+extern const bool missingFontFaceWarning;
+extern const bool glyphWarning;
+extern const bool shapingWarning;
+
+}
+
+}
+
+#endif