GetPrerequisites: Do not fail on files we cannot find

Historically we have always warned and skipped missing files.  Since
commit v3.4.0-rc1~264^2~1 (GetPrerequisites: Add error checks for
execute_process() calls, 2015-07-29) we fail instead, but this was not
an intentional part of that change.  Restore the warn-only behavior so
that missing system libraries (e.g. during cross-compiling) do not cause
failure.

Closes: #16523

Modules/GetPrerequisites.cmake

@@ -654,6 +654,8 @@ function(get_prerequisites target prerequisites_var exclude_system recurse exepa
 
   if(NOT EXISTS "${target}")
     message("warning: target '${target}' does not exist...")
+    set(${prerequisites_var} "" PARENT_SCOPE)
+    return()
   endif()
 
   set(gp_cmd_paths ${gp_cmd_paths}