configure: base GST_CXXFLAGS on --cflags from pkg-config
configure: base GST_CXXFLAGS on --cflags from pkg-config

pkg-config sets GST_CFLAGS and GST_LIBS. We need to use CFLAGS as a starting
point for for both C and CXX settings.

freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly,GStreamer/gst-plugins-ugly,jar1karp/gst-plugins-ugly,ahmedammar/platform_external_gst_plugins_ugly,Distrotech/gst-plugins-ugly,alessandrod/gst-plugins-ugly,krieger-od/gst-plugins-ugly,collects/gst-plugins-ugly,knuesel/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,Distrotech/gst-plugins-ugly,collects/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,krieger-od/gst-plugins-ugly,surround-io/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,shelsonjava/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,GStreamer/gst-plugins-ugly,fluendo/gst-plugins-ugly,ahmedammar/platform_external_gst_plugins_ugly,Lachann/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,cablelabs/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly,ylatuya/gst-plugins-ugly,Kurento/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,collects/gst-plugins-ugly,Kurento/gst-plugins-ugly,matsu/gst-plugins-ugly,sebras/gst-plugins-ugly,alessandrod/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly,shelsonjava/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-ugly,matsu/gst-plugins-ugly,krieger-od/gst-plugins-ugly,Distrotech/gst-plugins-ugly,matsu/gst-plugins-ugly,krieger-od/gst-plugins-ugly,GStreamer/gst-plugins-ugly,matsu/gst-plugins-ugly,collects/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,jar1karp/gst-plugins-ugly,Lachann/gst-plugins-ugly,knuesel/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,ylatuya/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,Distrotech/gst-plugins-ugly,jahrome/gst-plugins-ugly,jar1karp/gst-plugins-ugly,ahmedammar/platform_external_gst_plugins_ugly,Kurento/gst-plugins-ugly,fluendo/gst-plugins-ugly,jahrome/gst-plugins-ugly,Distrotech/gst-plugins-ugly,cablelabs/gst-plugins-ugly,jahrome/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,cablelabs/gst-plugins-ugly,sebras/gst-plugins-ugly,sebras/gst-plugins-ugly,Lachann/gst-plugins-ugly,surround-io/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-ugly,surround-io/gst-plugins-ugly,alessandrod/gst-plugins-ugly,fluendo/gst-plugins-ugly,shelsonjava/gst-plugins-ugly,ylatuya/gst-plugins-ugly,knuesel/gst-plugins-ugly