Explicitly mention, if call support is enabled at build time

fixes #366

CMakeLists.txt

@@ -446,6 +446,15 @@ endif()
 
 include(FindPkgConfig)
 pkg_check_modules(GSTREAMER IMPORTED_TARGET gstreamer-sdp-1.0>=1.14 gstreamer-webrtc-1.0>=1.14) 
+if (TARGET PkgConfig::GSTREAMER)
+	add_feature_info(voip ON "GStreamer found. Call support is enabled
+	automatically.")
+else()
+	add_feature_info(voip OFF "GStreamer could not be found on your system.
+	As a consequence call support has been disabled. If you don't want that,
+	make sure gstreamer-sdp-1.0>=1.16 gstreamer-webrtc-1.0>=1.16 can be found
+	via pkgconfig.")
+endif()
 
 # single instance functionality
 set(QAPPLICATION_CLASS QApplication CACHE STRING "Inheritance class for SingleApplication")