Guard testing code to avoid problems when including using FetchContent.MakeAvailable

CMakeLists.txt

@@ -2,7 +2,6 @@ cmake_minimum_required(VERSION 3.9)
 set(NAME "Vec3")
 project(${NAME})
 
-enable_testing()
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 add_compile_options(-Wall -Wextra -Wpedantic)
@@ -11,6 +10,9 @@ set(CMAKE_CXX_STANDARD 17)
 
 include_directories(include)
 add_subdirectory(include)
+
+if (PROJECT_IS_TOP_LEVEL)
+    enable_testing()
     add_subdirectory(tests)
 
     # Doxygen Build
@@ -36,3 +38,4 @@ if(DOXYGEN_FOUND)
     else(DOXYGEN_FOUND)
         message("Doxygen needs to be installed to generate the documentation.")
     endif(DOXYGEN_FOUND)
+endif()