Fix mistake in cmake variable check.
Fix mistake in cmake variable check.

This was discovered because this code path is used in the compilation tests.

iperry/qt-gstreamer,comicfans/libqtgstreamer,shadeslayer/qtgstreamer,knuesel/qt-gstreamer,comicfans/libqtgstreamer,tswindell/qt-gstreamer,tswindell/qt-gstreamer,ylatuya/qt-gstreamer,knuesel/qt-gstreamer,freedesktop-unofficial-mirror/gstreamer__qt-gstreamer,GStreamer/qt-gstreamer,GStreamer/qt-gstreamer,freedesktop-unofficial-mirror/gstreamer__qt-gstreamer,knuesel/qt-gstreamer,nasserGanjali/QtGStreamer,shadeslayer/qtgstreamer,cyndis/qtgstreamer,shadeslayer/qtgstreamer,iperry/qt-gstreamer,comicfans/libqtgstreamer,tswindell/qt-gstreamer,cyndis/qtgstreamer,detrout/qt-gstreamer,dejunk/gstreamer,dejunk/gstreamer,GStreamer/qt-gstreamer,freedesktop-unofficial-mirror/gstreamer__qt-gstreamer,ylatuya/qt-gstreamer,detrout/qt-gstreamer,iperry/qt-gstreamer,ylatuya/qt-gstreamer,detrout/qt-gstreamer,nasserGanjali/QtGStreamer,dejunk/gstreamer,nasserGanjali/QtGStreamer,cyndis/qtgstreamer