Improve backtraces on linux 64bit

On linux builds we force most debug symbols off in the WTF, JavaScriptCore
and WebCore submodules of QtWebKit. We do this to make linking possible
on 32bit but results in very poor backtraces from our users.

Since this is only needed on 32bit, we should disable it on 64bit.

Change-Id: I5009bb254471dc2213569b0cd011f283d6edfb95
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Fix build with GStreamer on non-linux X11 platforms

We detect GStreamer based on pkgconfig, this means it is not limited
to linux only, and we need to enable the same features everywhere it
is enabled.

Change-Id: I1b863c663e24f157c3f2d88548dc8c674dd63867
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>