Skip configuration of tests unless `BUILD_TESTING` is set

See https://github.com/Martchus/tagparser/issues/26
and https://cmake.org/cmake/help/latest/module/CTest.html

cmake/modules/TestTarget.cmake

@@ -8,6 +8,9 @@ if (TEST_CONFIG_DONE)
 endif ()
 
 include(TestUtilities)
+if(NOT BUILD_TESTING)
+    return()
+endif()
 
 # find and link against CppUnit if required (used by all my projects, so it is required by default)
 if (NOT META_NO_CPP_UNIT)

cmake/modules/TestUtilities.cmake

@@ -6,6 +6,9 @@ if (DEFINED TESTING_UTILITIES_LOADED)
 endif ()
 set(TESTING_UTILITIES_LOADED YES)
 
+# ensure CTest is loaded (e.g. for BUILD_TESTING variable)
+include(CTest)
+
 set(EXCLUDE_TEST_TARGET_BY_DEFAULT ON)
 if (ENABLE_DEVEL_DEFAULTS)
     set(EXCLUDE_TEST_TARGET_BY_DEFAULT OFF)