Merge pull request #12799 from alalek:update_build_js

* js: update build script

- support emscipten 1.38.12 (wasm is ON by default)
- verbose build messages

* js: use builtin Math functions

* js: disable tracing code completelly

modules/core/include/opencv2/core/utils/trace.hpp

@@ -37,6 +37,10 @@ namespace trace {
 //! @cond IGNORED
 #define CV_TRACE_NS cv::utils::trace
 
+#if !defined(OPENCV_DISABLE_TRACE) && defined(__EMSCRIPTEN__)
+#define OPENCV_DISABLE_TRACE 1
+#endif
+
 namespace details {
 
 #ifndef __OPENCV_TRACE