Update common cmake rules for documentation.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>
(cherry picked from commit ca3a88f7b8)

doc/CMakeLists.txt

@@ -1,9 +1,10 @@
 file( GLOB _dirs RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} * )
+list( REMOVE_ITEM _dirs html man )
+
 string( REGEX REPLACE "[ \r\n\t]+" ";" _linguas "$ENV{LINGUAS}" )
 
 foreach( _dir IN LISTS _dirs )
   if( IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/${_dir}
-      AND NOT "${_dir}" STREQUAL "man"
       AND ( "${_dir}" STREQUAL "en" OR
             "${_linguas}" MATCHES "^;*$" OR
             ";${_linguas};" MATCHES ";${_dir};" ))
@@ -25,6 +26,9 @@ foreach( _dir IN LISTS _dirs )
   endif()
 endforeach()
 
+if( EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/html/CMakeLists.txt )
+  add_subdirectory( html )
+endif()
 if( EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/man/CMakeLists.txt )
   add_subdirectory( man )
 endif()