don't use certain STL functions

some functions defined in <cmath>, as well as std::to_string aren't available on GNU's STL for some platforms, e.g. Android

src/csscolorparser/csscolorparser.cpp

@@ -116,7 +116,7 @@ const size_t namedColorCount = sizeof (namedColors) / sizeof (NamedColor);
 
 template <typename T>
 uint8_t clamp_css_byte(T i) {  // Clamp to integer 0 .. 255.
-    i = std::round(i);  // Seems to be what Chrome does (vs truncation).
+    i = ::round(i);  // Seems to be what Chrome does (vs truncation).
     return i < 0 ? 0 : i > 255 ? 255 : i;
 }