FindDoxygen: Ensure policy settings allow use of IN_LIST

In commit v3.9.0-rc1~55^2 (Improve Doxygen support, 2017-04-10)
use of the `IN_LIST` condition was added, but this is only supported
when policy CMP0057 is set to NEW.  Add a policy scope around the
module and enable the policy within it.  Otherwise it works only
in projects that happen to enable the policy.

Fixes: #18361

Modules/FindDoxygen.cmake

@@ -370,6 +370,9 @@ Deprecated Hint Variables
 
 #]=======================================================================]
 
+cmake_policy(PUSH)
+cmake_policy(SET CMP0057 NEW) # if IN_LIST
+
 # For backwards compatibility support
 if(Doxygen_FIND_QUIETLY)
     set(DOXYGEN_FIND_QUIETLY TRUE)
@@ -1108,3 +1111,5 @@ doxygen_add_docs() for target ${targetName}")
     )
 
 endfunction()
+
+cmake_policy(POP)