vlc: Fix build for Qt >= 5.7.0

This basically does something similar than the AUR build:

https://aur.archlinux.org/packages/vlc-qt5/

On our side, all there is to do is to force compiling using C++11 mode
and use a patch that the AUR package took from the following upstream
patchwork URL:

https://patches.videolan.org/patch/14061/



Instead of passing CXXFLAGS to the configure script, I'm using sed here
to make sure we don't override flags figured out by configure.

For example if ./configure is used with CXXFLAGS=-std=c++11 appended or
prepended, we have something like:

... -I../include   -std=c++11 -Wall -Wextra -Wsign-compare ...

While if we don't do that at all, we have something like:

... -I../include   -g -O2 -Wall -Wextra -Wsign-compare ...

Another way would be to use NIX_CFLAGS_COMPILE, but that would affect
even compilation of C code and thus resulting in a bunch of warnings
like this:

cc1: warning: command line option '-std=c++11' is valid for C++/ObjC++
              but not for C

So with our approach the flags during build look much better:

... -I../include   -std=c++11 -g -O2 -Wall -Wextra -Wsign-compare ...

Another thing I've changed is that the vlc_qt5 attribute in
all-packages.nix now uses the latest Qt 5 version, because the build for
Qt >= 5.7.0 is now no longer broken.

I've also ordered the preConfigure attribute before the configureFlags
attribute, because it makes more sense in terms of context (pre ->
configure -> post).

Tested by building on x86_64-linux with libsForQt56.vlc, libsForQt58.vlc
and vlc (the Qt 4 version, just to be sure I didn't accidentally break
it).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @ttuegel