Updates the cmake build system

It's still has some work, but it successfully builds on
all 3 main platforms.  Some of the outstanding items
include:

   Install target testing (mostly important for Linux)
   CMakeList clean up and comments
   Debug and Release build verification
   Audit of compile/link options
   Need a Mac signed and notarized build
   (and probably more)

help/CMakeLists.txt

@@ -0,0 +1,46 @@
+
+set( TARGET manual )
+set( TARGET_ROOT ${topdir}/help )
+
+message( STATUS "========== Configuring ${TARGET} ==========" )
+
+set( host "alphamanual.audacityteam.org" )
+set( src "https://${host}/man" )
+set( dst "$<TARGET_FILE_DIR:Audacity>/help/manual" )
+set( script_dir "${top_dir}/scripts/mw2html_audacity" )
+set( script "mw2html.py" )
+set( out_dir "${CMAKE_CURRENT_BINARY_DIR}" )
+set( out "${out_dir}/${host}/index.html" )
+
+if( CMAKE_SYSTEM_NAME MATCHES "Windows" )
+   file( TO_NATIVE_PATH
+      "${CMAKE_BINARY_DIR}/packages/${PYTHON_NAME}.${PYTHON_VERSION}/tools/python.exe"
+      python
+   )
+else()
+   find_package( Python2 )
+   if( Python2_FOUND )
+      set( python "${Python2_EXECUTABLE}" )
+   endif()
+endif()
+
+if( NOT DEFINED python )
+   message( WARNING "Python not found...unable to produce manual." )
+   return()
+endif()
+
+add_custom_command(
+   COMMENT
+      "Downloading manual from: ${src}"
+   COMMAND
+      "${python}" "${script_dir}/${script}" -s "${src}" "${out_dir}"
+   COMMAND
+      ${CMAKE_COMMAND} -E copy_directory "${out_dir}/${host}" "${dst}"
+   WORKING_DIRECTORY
+      "${script_dir}"
+   OUTPUT
+      "${out}"
+)
+
+add_custom_target( ${TARGET} DEPENDS "${out}" )
+add_dependencies( ${TARGET} Audacity )