emcc: error: wasm2js does not support source maps yet (debug in wasm for now)
このエラーはたぶんWASM=0 -g4にしてると出るはず。
-g3: When compiling to object files, keep debug info, including JS whitespace, function names, and LLVM debug info if any (this is the same as -g). -g4: When linking, generate a source map using LLVM debug information (which must be present in object files, i.e., they should have been compiled with -g).