-o flag is overwritten when used with CMake

Hey there,

The output flag "-o" for emcc seems to be overwritten when using together with CMake. Instead the target name is used.
As I post this here, I hope this is more an error of emscripten and not just me being stupid :)

Here is a minimal example that reproduces this inconvenience:

CMakeLists.txt

cmake_minimum_required(VERSION 3.5.0)
set(CMAKE_VERBOSE_MAKEFILE ON)
add_definitions(-std=c++11)
add_executable(targetname main.cpp)
set_target_properties(targetname PROPERTIES LINK_FLAGS "-o thing.js")

main.cpp

#include <stdio.h>

int main() {
  printf("hello, world!\n");
  return 0;
}

calling cmake:
cmake -DCMAKE_TOOLCHAIN_FILE=(..)/1.37.37/cmake/Modules/Platform/Emscripten.cmake .

cmake version is 3.5.1

When calling make (Version: GNU Make 4.1) some of the output looks like this:

[ 50%] Building CXX object CMakeFiles/targetname.dir/main.cpp.o
/home/wattux/src/emsdk/emscripten/1.37.37/em++ -std=c++11 -o CMakeFiles/targetname.dir/main.cpp.o -c /home/wattux/src/emscripten/main.cpp
[100%] Linking CXX executable targetname.js
/usr/bin/cmake -E cmake_link_script CMakeFiles/targetname.dir/link.txt --verbose=1
/home/wattux/src/emsdk/emscripten/1.37.37/em++     -o thing.js @CMakeFiles/targetname.dir/objects1.rsp  -o targetname.js 

In the last output line, one can see the second "-o" flag, which will apparently overwrite the first one (which is the correct behaviour, I'd say).

I looked at the toolchain file Emscripten.cmake and at line 188, 189 it states:

SET(CMAKE_C_CREATE_STATIC_LIBRARY "<CMAKE_C_COMPILER> -o <TARGET> <LINK_FLAGS> <OBJECTS>")
SET(CMAKE_CXX_CREATE_STATIC_LIBRARY "<CMAKE_CXX_COMPILER> -o <TARGET> <LINK_FLAGS> <OBJECTS>")

This is nothing but a wild guess, but maybe this will overwrite my own "-o" flag?

Big thanks in advance!

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