Here is what I did to enable Fortran support in Emscripten:

installed DraggonEgg plugin for gfortran (dragonegg.llvm.org)
introduced SHARED.DRAGONEGG with a path to dragonegg.so in the .emscripten config file
DRAGONEGG = os.path.expanduser('/usr/lib/gcc/x86_64-linux-gnu/4.6/plugin/dragonegg.so')
For the sake of clarity added "emfortran" (like em++) as a script for Fortan files
in the toolchain cmake/Platform/Emscripten.cmake, add "emfortran" as a compiler

(...)
if ("${CMAKE_C_COMPILER}" STREQUAL "")
    set(CMAKE_C_COMPILER "${EMSCRIPTEN_ROOT_PATH}/emcc${EMCC_SUFFIX}")
endif()
if ("${CMAKE_CXX_COMPILER}" STREQUAL "")
    set(CMAKE_CXX_COMPILER "${EMSCRIPTEN_ROOT_PATH}/em++${EMCC_SUFFIX}")
endif()
if ("${CMAKE_Fortran_COMPILER}" STREQUAL "")
    set(CMAKE_Fortran_COMPILER "${EMSCRIPTEN_ROOT_PATH}/emfortran${EMCC_SUFFIX}") # use our new script
endif()
(...)
include(CMakeForceCompiler)
CMAKE_FORCE_C_COMPILER("${CMAKE_C_COMPILER}" Clang)
CMAKE_FORCE_CXX_COMPILER("${CMAKE_CXX_COMPILER}" Clang)
CMAKE_FORCE_Fortran_COMPILER("${CMAKE_Fortran_COMPILER}" gfortran) # use gfortran with dragonegg
(...)
set(CMAKE_C_FLAGS_RELEASE "-DNDEBUG" CACHE STRING "Emscripten-overridden CMAKE_C_FLAGS_RELEASE")
set(CMAKE_C_FLAGS_MINSIZEREL "-DNDEBUG" CACHE STRING "Emscripten-overridden CMAKE_C_FLAGS_MINSIZEREL")
set(CMAKE_C_FLAGS_RELWITHDEBINFO "" CACHE STRING "Emscripten-overridden CMAKE_C_FLAGS_RELWITHDEBINFO")
set(CMAKE_CXX_FLAGS_RELEASE "-DNDEBUG" CACHE STRING "Emscripten-overridden CMAKE_CXX_FLAGS_RELEASE")
set(CMAKE_CXX_FLAGS_MINSIZEREL "-DNDEBUG" CACHE STRING "Emscripten-overridden CMAKE_CXX_FLAGS_MINSIZEREL")
set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "" CACHE STRING "Emscripten-overridden CMAKE_CXX_FLAGS_RELWITHDEBINFO")
set(CMAKE_Fortran_LINK_EXECUTABLE "") # don't link anything
(...)

finally edited "emcc" to parse Fortran files as well and added functionality to use gfortran and dragonegg

FORTRAN_ENDINGS = ('.f', '.F') # define the endings to identify a Fortran source file
CXX_SUFFIXES = ('.cpp', '.cxx', '.cc')
SOURCE_SUFFIXES = ('.c', '.cpp', '.cxx', '.cc', '.m', '.mm', '.f', '.FF') # accept .f and .FF files

here is the major change to emcc:

# First, generate LLVM bitcode. For each input file, we get base.o with bitcode
  for input_file in input_files:
    file_suffix = filename_type_suffix(input_file)
    if file_suffix.endswith(SOURCE_SUFFIXES):
      logging.debug('compiling source file: ' + input_file)
      input_file = shared.Building.preprocess(input_file, in_temp(uniquename(input_file)))
      output_file = in_temp(unsuffixed(uniquename(input_file)) + '.o')
      temp_files.append(output_file)
      fortran_llvm = in_temp(unsuffixed(uniquename(input_file)) + '.ll')
      if file_suffix.endswith(FORTRAN_ENDINGS): # create .ll (bitcode) file from .f (Fortran) files
        call    = 'gfortran'
        args    = [input_file, '-flto', '-S', '-m32', '-fplugin='+shared.DRAGONEGG, '-O2', '-o', fortran_llvm] # use the DRAGONEGG variable from the .emscripten config file
      else: 
        args = newargs + ['-emit-llvm', '-c', input_file, '-o', output_file]
        if file_suffix.endswith(CXX_SUFFIXES):
          args += shared.EMSDK_CXX_OPTS
      logging.debug("running: " + call + ' ' + ' '.join(args))
      execute([call] + args) # let compiler frontend print directly, so colors are saved (PIPE kills that)
      if file_suffix.endswith(FORTRAN_ENDINGS):
        shared.Building.llvm_as(fortran_llvm, output_file) # create .o file from .ll
      if not os.path.exists(output_file):
        logging.error('compiler frontend failed to generate LLVM bitcode, halting')
        sys.exit(1)
    else: # bitcode
        (...)

I know that this patch is a very hacky one. It would be great if someone could present a similiar solution that can be merged into the official repo!

Fortran support #998

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