FindBoost: Simplify search in lists

Instead of `list(FIND...)` and then checking result for `-1`
(found/not-found), nowadays `if` command has the `IN_LIST` test for
that.

This change was originally made by commit v3.9.0-rc1~41^2 (FindBoost:
Simplify search in lists, 2017-04-23) but then had to be reverted by
commit v3.9.2~3^2 (FindBoost: Revert "Simplify search in lists.",
2017-09-05) due to problems related to using `find_dependency`.  Those
problems were addressed by commit 3080a0a611 (FindBoost: Improve
behavior when thread dependency is missing, 2017-09-15), so now we can
restore the original change.

Issue: #17252

Modules/FindBoost.cmake

@@ -214,6 +214,10 @@
 #
 # Set Boost_NO_BOOST_CMAKE to ON to disable the search for boost-cmake.
 
+# Save project's policies
+cmake_policy(PUSH)
+cmake_policy(SET CMP0057 NEW) # if IN_LIST
+
 #-------------------------------------------------------------------------------
 # Before we go searching, check whether boost-cmake is available, unless the
 # user specifically asked NOT to search for boost-cmake.
@@ -905,9 +909,7 @@ function(_Boost_MISSING_DEPENDENCIES componentvar extravar)
       set(_Boost_${uppercomponent}_DEPENDENCIES ${_Boost_${uppercomponent}_DEPENDENCIES} PARENT_SCOPE)
       set(_Boost_IMPORTED_TARGETS ${_Boost_IMPORTED_TARGETS} PARENT_SCOPE)
       foreach(componentdep ${_Boost_${uppercomponent}_DEPENDENCIES})
-        list(FIND _boost_processed_components "${componentdep}" _boost_component_found)
-        list(FIND _boost_new_components "${componentdep}" _boost_component_new)
-        if (_boost_component_found EQUAL -1 AND _boost_component_new EQUAL -1)
+        if (NOT ("${componentdep}" IN_LIST _boost_processed_components OR "${componentdep}" IN_LIST _boost_new_components))
           list(APPEND _boost_new_components ${componentdep})
         endif()
       endforeach()
@@ -1541,8 +1543,7 @@ endif()
 _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)
-if(NOT _Boost_THREAD_DEPENDENCY_LIBS EQUAL -1)
+if("thread" IN_LIST Boost_FIND_COMPONENTS)
   if(Boost_FIND_QUIETLY)
     set(_Boost_find_quiet QUIET)
   else()
@@ -1977,3 +1978,6 @@ list(REMOVE_DUPLICATES _Boost_COMPONENTS_SEARCHED)
 list(SORT _Boost_COMPONENTS_SEARCHED)
 set(_Boost_COMPONENTS_SEARCHED "${_Boost_COMPONENTS_SEARCHED}"
   CACHE INTERNAL "Components requested for this build tree.")
+
+# Restore project's policies
+cmake_policy(POP)