Adjust EM_BUILD_VERBOSE environment variable to take values 0,1,2 or 3, with the following meanings:

0 - No verbose build. Emscripten will mute stdout and stderr invokations of external tools (configure, cmake, make). Stdout and stderr of those runs will be logged to file (the old mechanism)
1 - Print stderr.
2 - Print stderr and stdout.
3 - Print stderr and stdout, and invoke make with VERBOSE=1.