gstpluginloader: when env var is set do not fall through to system plugin scanner
If we set a custom GST_PLUGIN_SCANNER env var, then we probably want to use that and only that. Falling through to the one installed on the system is problamatic in cross-compilation environemnts, regardless of whether one pointed to by the env var succeeded or failed. taken from: http://cgit.openembedded.org/openembedded-core/commit/meta/recipes-multimedia/gstreamer/gstreamer1.0/0001-gst-gstpluginloader.c-when-env-var-is-set-do-not-fal.patch?id=0db7ba34ca41b107042306d13a6f0162885c123b Part-of: <!669>