Tests: Execute compile features tests unconditionally.

Conditionally create a dummy test if there are no known features.

Tests/CMakeCommands/target_compile_features/CMakeLists.txt

@@ -1,6 +1,14 @@
 cmake_minimum_required(VERSION 3.0)
 project(target_compile_features)
 
+if (NOT CMAKE_CXX_COMPILE_FEATURES)
+  file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/test_dummy.cpp"
+    "int main(int,char**) { return 0; }\n"
+  )
+  add_executable(target_compile_features "${CMAKE_CURRENT_BINARY_DIR}/test_dummy.cpp")
+  return()
+endif()
+
 set(CMAKE_VERBOSE_MAKEFILE ON)
 
 add_executable(target_compile_features main.cpp)