KaiOS: include extra copyright notices in manual

The KaiOS build includes compiled versions of various Emscripten library
files.  These are generally under the MIT licence like Puzzles itself.
The MIT licence requires that the licence, and the copyright notice, be
"included in all copies or substantial portions of the Software."

Since each KaiOS package includes the full manual, which already
contains the licence for Puzzles itself, adding the copyright notices
there seems like the best approach.  I've done this by providing an
additional input file that contains the licences for source files used
by a current Emscripten build.  More automation might be nice, but the
set of copyright notices is unlikely to change very much.  There are
basically one for Emscripten, one for musl, and a few for odd bits of
third-party code embedded in musl.

cmake/platforms/emscripten.cmake

@@ -73,9 +73,11 @@ function(build_platform_extras)
     add_custom_command(OUTPUT ${help_dir}/en/index.html
       COMMAND ${HALIBUT} --html -Chtml-template-fragment:%k
         ${CMAKE_CURRENT_SOURCE_DIR}/puzzles.but
+        ${CMAKE_CURRENT_SOURCE_DIR}/emcccopy.but
       DEPENDS
       ${help_dir}/en
       ${CMAKE_CURRENT_SOURCE_DIR}/puzzles.but
+      ${CMAKE_CURRENT_SOURCE_DIR}/emcccopy.but
       WORKING_DIRECTORY ${help_dir}/en)
     add_custom_target(kaios_help ALL
       DEPENDS ${help_dir}/en/index.html)