update CMake documentation build for recent changes to the html doc build procedure

###############################################################################

# Build documentation

###############################################################################

option(BUILD_DOC "Build LAMMPS documentation" OFF)

option(BUILD_DOC "Build LAMMPS HTML documentation" OFF)

if(BUILD_DOC)

  include(ProcessorCount)

  ProcessorCount(NPROCS)

  find_package(PythonInterp 3 REQUIRED)

  set(VIRTUALENV ${PYTHON_EXECUTABLE} -m virtualenv)

    COMMAND ${DOCENV_BINARY_DIR}/pip install --upgrade ${LAMMPS_DOC_DIR}/utils/converters

  )

  # download mathjax distribution and unpack to folder "mathjax"

  file(DOWNLOAD "https://github.com/mathjax/MathJax/archive/3.0.5.tar.gz"

    "${CMAKE_CURRENT_BINARY_DIR}/mathjax.tar.gz"

    EXPECTED_MD5 5d9d3799cce77a1a95eee6be04eb68e7)

  if(NOT EXISTS ${CMAKE_CURRENT_BINARY_DIR}/mathjax)

    execute_process(COMMAND ${CMAKE_COMMAND} -E tar xzf mathjax.tar.gz WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})

    file(GLOB MATHJAX_VERSION_DIR ${CMAKE_CURRENT_BINARY_DIR}/MathJax-*)

    execute_process(COMMAND ${CMAKE_COMMAND} -E rename ${MATHJAX_VERSION_DIR} ${CMAKE_CURRENT_BINARY_DIR}/mathjax)

  endif()

  # note, this may run in parallel with other tasks, so we must not use multiple processes here

  add_custom_command(

    OUTPUT html

    DEPENDS ${DOC_SOURCES} docenv requirements.txt

    COMMAND ${DOCENV_BINARY_DIR}/sphinx-build -j ${NPROCS} -b html -c ${LAMMPS_DOC_DIR}/utils/sphinx-config -d ${CMAKE_BINARY_DIR}/doctrees ${LAMMPS_DOC_DIR}/src html

    COMMAND ${DOCENV_BINARY_DIR}/sphinx-build -b html -c ${LAMMPS_DOC_DIR}/utils/sphinx-config -d ${CMAKE_BINARY_DIR}/doctrees ${LAMMPS_DOC_DIR}/src html

  )

  # copy extra images for link targets

  add_custom_command(

    OUTPUT html/JPG

    DEPENDS html

    COMMAND ${CMAKE_COMMAND} -E make_directory html/JPG

  )

  set(HTML_EXTRA_IMAGES balance_nonuniform.jpg balance_rcb.jpg

    balance_uniform.jpg bow_tutorial_01.png bow_tutorial_02.png

    bow_tutorial_03.png bow_tutorial_04.png bow_tutorial_05.png

    dump1.jpg dump2.jpg examples_mdpd.gif gran_funnel.png gran_mixer.png

    hop1.jpg hop2.jpg saed_ewald_intersect.jpg saed_mesh.jpg

    screenshot_atomeye.jpg screenshot_gl.jpg screenshot_pymol.jpg

    screenshot_vmd.jpg sinusoid.jpg xrd_mesh.jpg)

  set(HTML_IMAGE_TARGETS "")

  foreach(_IMG ${HTML_EXTRA_IMAGES})

    string(PREPEND _IMG JPG/)

    list(APPEND HTML_IMAGE_TARGETS "html/${_IMG}")

    add_custom_command(

      OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/html/${_IMG}

      DEPENDS ${LAMMPS_DOC_DIR}/src/${_IMG} html/JPG

      COMMAND ${CMAKE_COMMAND} -E copy ${LAMMPS_DOC_DIR}/src/${_IMG} ${CMAKE_BINARY_DIR}/html/${_IMG}

    )

  endforeach()

  # copy mathjax tree

  add_custom_command(

    OUTPUT html/_static/mathjax

    DEPENDS html

    COMMAND ${CMAKE_COMMAND} -E make_directory html/_static/mathjax

  )

  add_custom_command(

    OUTPUT html/_static/mathjax/es5

    DEPENDS html/_static/mathjax

    COMMAND ${CMAKE_COMMAND} -E copy_directory ${CMAKE_BINARY_DIR}/mathjax/es5 html/_static/mathjax/es5

  )

  add_custom_target(

    doc ALL

    DEPENDS html

    DEPENDS html html/JPG html/_static/mathjax/es5 ${HTML_IMAGE_TARGETS}

    SOURCES ${LAMMPS_DOC_DIR}/utils/requirements.txt ${DOC_SOURCES}

  )