FindBoost: Tolerate missing indirect dependencies (#16013)

Depending upon the configuration, certain components may or may not
be installed, for example Boost.Regex, but other components may
still have header dependencies upon these components which will
obviously fail to work.  Since we can't make a sensible
determination with the hardcoded dependency information, we
choose to interpret these dependencies less strictly.

Modules/FindBoost.cmake

@@ -745,9 +745,10 @@ endfunction()
 # defined; FALSE if dependency information is unavailable).
 #
 # componentvar - the component list variable name
+# extravar - the indirect dependency list variable name
 #
 #
-function(_Boost_MISSING_DEPENDENCIES componentvar)
+function(_Boost_MISSING_DEPENDENCIES componentvar extravar)
   # _boost_unprocessed_components - list of components requiring processing
   # _boost_processed_components - components already processed (or currently being processed)
   # _boost_new_components - new components discovered for future processing
@@ -773,7 +774,12 @@ function(_Boost_MISSING_DEPENDENCIES componentvar)
     set(_boost_unprocessed_components ${_boost_new_components})
     unset(_boost_new_components)
   endwhile()
+  set(_boost_extra_components ${_boost_processed_components})
+  if(_boost_extra_components AND ${componentvar})
+    list(REMOVE_ITEM _boost_extra_components ${${componentvar}})
+  endif()
   set(${componentvar} ${_boost_processed_components} PARENT_SCOPE)
+  set(${extravar} ${_boost_extra_components} PARENT_SCOPE)
 endfunction()
 
 #
@@ -1306,7 +1312,7 @@ endif()
 
 # Additional components may be required via component dependencies.
 # Add any missing components to the list.
-_Boost_MISSING_DEPENDENCIES(Boost_FIND_COMPONENTS)
+_Boost_MISSING_DEPENDENCIES(Boost_FIND_COMPONENTS _Boost_EXTRA_FIND_COMPONENTS)
 
 # If thread is required, get the thread libs as a dependency
 list(FIND Boost_FIND_COMPONENTS thread _Boost_THREAD_DEPENDENCY_LIBS)
@@ -1484,6 +1490,10 @@ if(Boost_FOUND)
       list(APPEND _Boost_MISSING_COMPONENTS ${COMPONENT})
     endif()
   endforeach()
+  if(_Boost_MISSING_COMPONENTS AND _Boost_EXTRA_FIND_COMPONENTS)
+    # Optional indirect dependencies are not counted as missing.
+    list(REMOVE_ITEM _Boost_MISSING_COMPONENTS ${_Boost_EXTRA_FIND_COMPONENTS})
+  endif()
 
   if(Boost_DEBUG)
     message(STATUS "[ ${CMAKE_CURRENT_LIST_FILE}:${CMAKE_CURRENT_LIST_LINE} ] Boost_FOUND = ${Boost_FOUND}")