avoid conflicts with catch already pulled via submodule in hosting project

Author: Henry Hoegelow

tests/CMakeLists.txt

@@ -1,17 +1,19 @@
 cmake_minimum_required(VERSION 3.16.3)
 
-Include(FetchContent)
+if(NOT TARGET Catch2WithMain)
+    Include(FetchContent)
 
-FetchContent_Declare(
-    Catch2
-    GIT_REPOSITORY https://github.com/catchorg/Catch2.git
-    GIT_TAG v3.4.0 # or a later release
-)
+    FetchContent_Declare(
+        Catch2
+        GIT_REPOSITORY https://github.com/catchorg/Catch2.git
+        GIT_TAG v3.4.0 # or a later release
+    )
 
-FetchContent_MakeAvailable(Catch2)
-list(APPEND CMAKE_MODULE_PATH ${catch2_SOURCE_DIR}/extras)
-include(CTest)
-include(Catch)
+    FetchContent_MakeAvailable(Catch2)
+    list(APPEND CMAKE_MODULE_PATH ${catch2_SOURCE_DIR}/extras)
+    include(CTest)
+    include(Catch)
+endif()
 
 add_executable(reactive-tests
     LatchTests.cpp