Without this, configure will say:
Checking for openal... no
/tmp/nix-build-qtmultimedia-5.5.0.drv-0/qtmultimedia-opensource-src-5.5.0/qtmultimedia.pro:28:
Variable GST_VERSION is not defined.
Checking for resourcepolicy... no
And there may be application runtime errors like
defaultServiceProvider::requestService(): no service found for - "org.qt-project.qt.camera"
After this fix, configure will say:
Checking for openal... no
Checking for gstreamer... yes
Checking for gstreamer_photography... no
Checking for gstreamer_encodingprofiles... yes
Checking for gstreamer_appsrc... yes
Checking for linux_v4l... yes
Checking for resourcepolicy... no
And the above runtime error will not appear.
This fix is similar to 449b6028 ("qt5.multimedia: fix gstreamer
support."), except with Qt 5.5 we also need to set GST_VERSION.
