reproc: Emscripten is not supported with multithread (pthread flags is set on example but not on the library)

Resulting ld bug:

> wasm-ld: error: 'atomics' feature is used by d1_lib.o, so --shared-memory 
> must be used
>
> This one means that `dl_lib.o` was compiled with -pthread already 
> (-pthread enables the 'atomics' feature), so its atomic operations have 
> been lowered to real atomic WebAssembly instructions. However, because 
> you're not passing -pthread at link time, the linker tries to produce a 
> module with an unshared memory, which would make those atomic instructions 
> invalid.
>
> wasm-ld: error: Target feature 'atomics' used in d1_lib.o is disallowed by 
> ConcurrentScheduler.cpp.o. Use --no-check-features to suppress.
>
> This line tells us that ConcurrentScheduler.cpp.o was compiled without 
> -pthread, which means its atomic operations were lowered to non-atomic 
> WebAssembly instruction because the atomics feature was not enabled. That 
> means that ConcurrentScheduler.cpp.o is not safe to link with dl_lib.o 
> because the resulting program would not be thread-safe, even if the program 
> was thread-safe at the source level. When using the LLVM backend, the 
> wasm-ld linker helpfully recognizes this error and does not let you link an 
> unsafe program. Fastcomp would just silently link your program into a 
> thread-unsafe binary.