title: Issue specifying option while using Emscripten (Emcmake)
body: <p>My question is regarding using a <code>CMakeLists.txt</code> with Emscriptem and specifying the output type along with a command line option.</p>
  
  <p>I want to take a simple Emscripten command such as: <code>emcc file.cpp -o file.html --preload-file asset_dir/</code> and change it to something that I can specify within my CMake system. I tried the naive approach of renaming the executable to have an extension of html but that didn't work. I also tried using <code>-D--preload-file:PATH=asset_dir</code> and that did not work either.</p>
  
  <p>My <code>CMakeLists.txt</code> file is small and is contained below. I use the command <code>emcmake cmake -G"Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=em++ .. &amp;&amp; emmake make</code> to build it.</p>
  
  <pre><code>CMAKE_MINIMUM_REQUIRED(VERSION 3.2.0 FATAL_ERROR)
  PROJECT(ProjJS)
  
  # Set typical CMAKE settings
  SET(CMAKE_BUILD_TYPE_INIT "Release")
  SET(CMAKE_VERBOSE_MAKEFILE OFF CACHE BOOL "Turn on Verbose Makefiles" FORCE)
  set(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/modules")
  
  SET(CMAKE_CXX_STANDARD 11)
  SET(CMAKE_CXX_STANDARD_REQUIRED ON)
  
  SET(PROJ_SOURCES
      hello.cpp
      ....
  )
  
  set(NAME_OF_EXE "ProjJS")
  
  set(BOOST_LIB "boost")
  set(BOOST_INCLUDE_PATH "${CMAKE_SOURCE_DIR}/extlibs/")
  set(BOOST_LIB_PATH "${CMAKE_SOURCE_DIR}/extlibs/boost/libs/")
  
  add_subdirectory(extlibs/boost)
  
  
  SET(BOOST_PROGRAM_OPTIONS_SOURCES
      ${BOOST_LIB_PATH}/program_options/cmdline.cpp
      ${BOOST_LIB_PATH}/program_options/config_file.cpp
      ....
  )
  
  SET(BOOST_SYSTEM_SOURCES
      ${BOOST_LIB_PATH}/system/error_code.cpp
  )
  
  ADD_EXECUTABLE(${NAME_OF_EXE} ${PROJ_SOURCES})
  add_library(${BOOST_LIB} STATIC ${BOOST_PROGRAM_OPTIONS_SOURCES} ${BOOST_SYSTEM_SOURCES})
  TARGET_INCLUDE_DIRECTORIES(${BOOST_LIB} PUBLIC "${BOOST_INCLUDE_PATH}")
  
  TARGET_LINK_LIBRARIES(${NAME_OF_EXE} PUBLIC ${BOOST_LIB})
  TARGET_INCLUDE_DIRECTORIES(${NAME_OF_EXE} BEFORE PRIVATE "${CMAKE_SOURCE_DIR}/include")
  TARGET_INCLUDE_DIRECTORIES(${NAME_OF_EXE} BEFORE PRIVATE "${BOOST_INCLUDE_PATH}" ${PROJ_SOURCES})
  </code></pre>