Tests: Fix GTK2Components.gtkmm test with sigc++ >= 2.5.1

FindGTK2 adds compile features to GTK2::sigc++ so that clients will
compile as C++11 or above.  However, our test case covers using just the
library list variables instead of the imported targets.  Fix this case
by propagating the compile features manually.

Tests/FindGTK2/gtkmm/CMakeLists.txt

@@ -12,3 +12,8 @@ target_link_libraries(gtkmm-target GTK2::gtkmm)
 add_executable(gtkmm-all-libs WIN32 main.cpp helloworld.cpp helloworld.h)
 target_link_libraries(gtkmm-all-libs ${GTK2_LIBRARIES})
 target_include_directories(gtkmm-all-libs PRIVATE ${GTK2_INCLUDE_DIRS})
+
+# Linking via the library variables does not cause compile feature
+# requirements to propagate.  Do it manually for purposes of this test.
+get_property(features TARGET GTK2::sigc++ PROPERTY INTERFACE_COMPILE_FEATURES)
+set_property(TARGET gtkmm-all-libs PROPERTY COMPILE_FEATURES ${features})