No option in build.c to enable simdutf support

[tosanov]

Hello, and thank you for your work on this project.

I noticed that support for simdutf was added to the C++ source code a couple of months ago. This is a great performance feature.
However, it seems the build.c script was not updated at the time to include a corresponding build flag. As a result, there is currently no way to define the UWS_USE_SIMDUTF macro and compile the library with this optimization enabled.

It would be very helpful if an option could be added to build.c to support this feature.

Thanks!

[uNetworkingAB]

CXXFLAGS=-DUWS_USE_SIMDUTF make