Summary says it all, I was working today to build Qt 5.5.1 stack on CentOS/RHEL 6, and was largely successful, except for qtmultimedia.

RHEL6 currently ships with gstreamer-0.10.29, and qtmultimedia-5.5.x started using some api's added only in 0.10.30 and 0.10.31.

Those I've found so far:

• in src/gsttools/qgstutils.cpp:
  GST_VIDEO_FORMAT_RGB16
  added in 0.10.30

gst_date_time_new_local_time api
added in 0.10.31

• in src/gsttools/qgstreamervideooverlay.cpp:
  gst_element_factory_list_get_elements (and related constants) api
  added in 0.10.31

*in src/plugins/gstreamer/audiodecoder/qgstreameraudiodecoderserviceplugin.cpp:
src/plugins/gstreamer/mediaplayer/qgstreamerplayerserviceplugin.cpp:
src/plugins/gstreamer/mediacapture/qgstreamercaptureserviceplugin.cpp
gst_element_factory_list_is_type (and related constants) api
added in 0.10.31

I'll attach a naive patch that simply avoids the use of these to allow building to continue, but that's probably far from ideal.

Also, looks like the build proceeds ok if gstreamer headers are not present (with reduced functionality I assume).

So way forward: either make things work (somehow) with 0.10.29, or raise minimal requirement to 0.10.31 (and ideally add a check for it).
http://doc.qt.io/qt-5/linux-requirements.html currently only mentions "The minimum required version of GStreamer is 0.10."