Make Catch2 discovery more robust

CMakeLists.txt

@@ -120,7 +120,9 @@ SET_SOURCE_FILES_PROPERTIES( "./source/src/ann/kd_pr_search.cpp" PROPERTIES COMP
 #ADD_SHARED_LIBRARY( ${PROJECT_NAME}_ann ${ann_library_sources} )
 #INSTALL_SHARED_LIBRARY( ${PROJECT_NAME}_ann DESTINATION lib )
 
-option(USE_EXTERNAL_CATCH2 "Link against an external Catch2 v3 static library, otherwise build it locally" ON)
+option(USE_EXTERNAL_CATCH2 "Link against an external Catch2 v3 static library, otherwise build it locally" AUTO)
+set(USE_EXTERNAL_CATCH2 AUTO CACHE STRING "Link against an external Catch2 v3 static library, otherwise build it locally")
+set_property(CACHE USE_EXTERNAL_CATCH2 PROPERTY STRINGS AUTO ON OFF)
 
 IF( BUILD_TESTING )
   ADD_SUBDIRECTORY(source/tests)

source/tests/CMakeLists.txt

@@ -11,9 +11,20 @@ ENDMACRO()
 ADD_MARLINUTIL_TEST( testmarlinutil )
 
 #--- Unit test setup. Use existing Catch2 or fetch a suitable version and build it
-if(USE_EXTERNAL_CATCH2)
-  FIND_PACKAGE(Catch2 3.0.0 REQUIRED)
+if(CMAKE_CXX_STANDARD GREATER_EQUAL 20)
+  set(CATCH2_MIN_VERSION 3.4)
 else()
+  set(CATCH2_MIN_VERSION 3.1)
+endif()
+if(USE_EXTERNAL_CATCH2)
+  if (USE_EXTERNAL_CATCH2 STREQUAL AUTO)
+    find_package(Catch2 ${CATCH2_MIN_VERSION})
+  else()
+    find_package(Catch2 ${CATCH2_MIN_VERSION} REQUIRED)
+  endif()
+endif()
+
+if(NOT Catch2_FOUND)
   MESSAGE(STATUS "Fetching local copy of Catch2 library for unit-tests...")
   # Build Catch2 with the default flags, to avoid generating warnings when we
   # build it