FindBoost: Do not add any Boost targets until after Boost found

Move creation of the "helper" targets that do not require anything to be
found to be done only after Boost is found.

Fixes: #20261

Modules/FindBoost.cmake

@@ -1470,43 +1470,6 @@ _Boost_DEBUG_PRINT_VAR("${CMAKE_CURRENT_LIST_FILE}" "${CMAKE_CURRENT_LIST_LINE}"
 _Boost_DEBUG_PRINT_VAR("${CMAKE_CURRENT_LIST_FILE}" "${CMAKE_CURRENT_LIST_LINE}" "Boost_ADDITIONAL_VERSIONS")
 _Boost_DEBUG_PRINT_VAR("${CMAKE_CURRENT_LIST_FILE}" "${CMAKE_CURRENT_LIST_LINE}" "Boost_NO_SYSTEM_PATHS")
 
-# Supply Boost_LIB_DIAGNOSTIC_DEFINITIONS as a convenience target. It
-# will only contain any interface definitions on WIN32, but is created
-# on all platforms to keep end user code free from platform dependent
-# code.  Also provide convenience targets to disable autolinking and
-# enable dynamic linking.
-if(NOT TARGET Boost::diagnostic_definitions)
-  add_library(Boost::diagnostic_definitions INTERFACE IMPORTED)
-  add_library(Boost::disable_autolinking INTERFACE IMPORTED)
-  add_library(Boost::dynamic_linking INTERFACE IMPORTED)
-  set_target_properties(Boost::dynamic_linking PROPERTIES
-    INTERFACE_COMPILE_DEFINITIONS "BOOST_ALL_DYN_LINK")
-endif()
-if(WIN32)
-  # In windows, automatic linking is performed, so you do not have
-  # to specify the libraries.  If you are linking to a dynamic
-  # runtime, then you can choose to link to either a static or a
-  # dynamic Boost library, the default is to do a static link.  You
-  # can alter this for a specific library "whatever" by defining
-  # BOOST_WHATEVER_DYN_LINK to force Boost library "whatever" to be
-  # linked dynamically.  Alternatively you can force all Boost
-  # libraries to dynamic link by defining BOOST_ALL_DYN_LINK.
-
-  # This feature can be disabled for Boost library "whatever" by
-  # defining BOOST_WHATEVER_NO_LIB, or for all of Boost by defining
-  # BOOST_ALL_NO_LIB.
-
-  # If you want to observe which libraries are being linked against
-  # then defining BOOST_LIB_DIAGNOSTIC will cause the auto-linking
-  # code to emit a #pragma message each time a library is selected
-  # for linking.
-  set(Boost_LIB_DIAGNOSTIC_DEFINITIONS "-DBOOST_LIB_DIAGNOSTIC")
-  set_target_properties(Boost::diagnostic_definitions PROPERTIES
-    INTERFACE_COMPILE_DEFINITIONS "BOOST_LIB_DIAGNOSTIC")
-  set_target_properties(Boost::disable_autolinking PROPERTIES
-    INTERFACE_COMPILE_DEFINITIONS "BOOST_ALL_NO_LIB")
-endif()
-
 cmake_policy(GET CMP0074 _Boost_CMP0074)
 if(NOT "x${_Boost_CMP0074}x" STREQUAL "xNEWx")
   _Boost_CHECK_SPELLING(Boost_ROOT)
@@ -2299,6 +2262,43 @@ if(Boost_FOUND)
       endif()
     endif()
   endforeach()
+
+  # Supply Boost_LIB_DIAGNOSTIC_DEFINITIONS as a convenience target. It
+  # will only contain any interface definitions on WIN32, but is created
+  # on all platforms to keep end user code free from platform dependent
+  # code.  Also provide convenience targets to disable autolinking and
+  # enable dynamic linking.
+  if(NOT TARGET Boost::diagnostic_definitions)
+    add_library(Boost::diagnostic_definitions INTERFACE IMPORTED)
+    add_library(Boost::disable_autolinking INTERFACE IMPORTED)
+    add_library(Boost::dynamic_linking INTERFACE IMPORTED)
+    set_target_properties(Boost::dynamic_linking PROPERTIES
+      INTERFACE_COMPILE_DEFINITIONS "BOOST_ALL_DYN_LINK")
+  endif()
+  if(WIN32)
+    # In windows, automatic linking is performed, so you do not have
+    # to specify the libraries.  If you are linking to a dynamic
+    # runtime, then you can choose to link to either a static or a
+    # dynamic Boost library, the default is to do a static link.  You
+    # can alter this for a specific library "whatever" by defining
+    # BOOST_WHATEVER_DYN_LINK to force Boost library "whatever" to be
+    # linked dynamically.  Alternatively you can force all Boost
+    # libraries to dynamic link by defining BOOST_ALL_DYN_LINK.
+
+    # This feature can be disabled for Boost library "whatever" by
+    # defining BOOST_WHATEVER_NO_LIB, or for all of Boost by defining
+    # BOOST_ALL_NO_LIB.
+
+    # If you want to observe which libraries are being linked against
+    # then defining BOOST_LIB_DIAGNOSTIC will cause the auto-linking
+    # code to emit a #pragma message each time a library is selected
+    # for linking.
+    set(Boost_LIB_DIAGNOSTIC_DEFINITIONS "-DBOOST_LIB_DIAGNOSTIC")
+    set_target_properties(Boost::diagnostic_definitions PROPERTIES
+      INTERFACE_COMPILE_DEFINITIONS "BOOST_LIB_DIAGNOSTIC")
+    set_target_properties(Boost::disable_autolinking PROPERTIES
+      INTERFACE_COMPILE_DEFINITIONS "BOOST_ALL_NO_LIB")
+  endif()
 endif()
 
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