unix, gtk: Install and use HTML help

- Generate HTML pages from the manual, and install them
- Add "Contents" and "Help on <name>" menu items that will open the
  appropriate page in a web browser

cmake/platforms/unix.cmake

@@ -12,6 +12,11 @@ in a crowded bin directory, e.g. \"sgt-\"")
 
 find_package(PkgConfig REQUIRED)
 
+find_program(HALIBUT halibut)
+if(NOT HALIBUT)
+  message(WARNING "HTML documentation cannot be built (did not find halibut)")
+endif()
+
 set(PUZZLES_GTK_FOUND FALSE)
 macro(try_gtk_package VER PACKAGENAME)
   if(NOT PUZZLES_GTK_FOUND AND
@@ -52,6 +57,8 @@ if(STRICT AND (CMAKE_C_COMPILER_ID MATCHES "GNU" OR
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wwrite-strings -std=c99 -pedantic -Werror")
 endif()
 
+add_compile_definitions(HELP_DIR="${CMAKE_INSTALL_PREFIX}/share/sgt-puzzles/help")
+
 function(get_platform_puzzle_extra_source_files OUTVAR NAME)
   if(build_icons AND EXISTS ${CMAKE_SOURCE_DIR}/icons/${NAME}.sav)
     # If we have the equipment to rebuild the puzzles' icon images
@@ -107,4 +114,20 @@ function(set_platform_puzzle_target_properties NAME TARGET)
 endfunction()
 
 function(build_platform_extras)
+  if(HALIBUT)
+    set(help_dir ${CMAKE_CURRENT_BINARY_DIR}/help)
+    add_custom_command(OUTPUT ${help_dir}/en
+      COMMAND ${CMAKE_COMMAND} -E make_directory ${help_dir}/en)
+    add_custom_command(OUTPUT ${help_dir}/en/index.html
+      COMMAND ${HALIBUT} --html ${CMAKE_CURRENT_SOURCE_DIR}/puzzles.but
+      DEPENDS
+      ${help_dir}/en
+      ${CMAKE_CURRENT_SOURCE_DIR}/puzzles.but
+      WORKING_DIRECTORY ${help_dir}/en)
+    add_custom_target(unix_help ALL
+      DEPENDS ${help_dir}/en/index.html)
+
+    install(DIRECTORY ${help_dir}
+      DESTINATION share/sgt-puzzles)
+  endif()
 endfunction()