On cross-compiling qml extensions, qt packaging needs changing

[Dmitrijs Ledkovs]

So at the moment I have hacked up a way to cross compile qml extensions [1]
The solution is not as fluid and packaging changes are needed in qt*
packages to make all of this nicer.

Foremost one cannot install ubuntu-sdk for multiple architectures:
$ apt-get install ubuntu-sdk ubuntu-sdk:armhf

Even a more simple case of:
$ apt-get install qtbase5-dev qtbase5-dev:armhf

Doesn't install because of GL somewhere down the stack:
 qtbase5-dev : Depends: libgl1-mesa-dev but it is not going to be installed or
                        libgl-dev
               Depends: libglu1-mesa-dev but it is not going to be installed or
                        libglu-dev
E: Unable to correct problems, you have h

If you want to experiment with above, enabling armhf is easy:
$ dpkg --add-architecture
Add:
deb [arch=armhf] http://ports.ubuntu.com/ubuntu-ports saucy main
restricted universe multiverse

To sources.list (if you want to get rid of warnings, other
repositories should list which architectures you want from them, e.g.
[arch=amd64,i386] for nomal ubuntu mirrors)

Therefore at the moment, I create a chroot where I have most of
-dev:armhf packages & cross-toolchain installed, whilst on the host
system is where I install qmake & generate the makefiles.

I've hacked up qmake mkspec files which to honour dpkg-architecture
fields which is the standard way to declare native or cross
compilations on Ubuntu. the variables from dpkg-architecutre can be
exported in the environment, or one can use it as a wrapper script:
dpkg-architecture -aarmhf -c qmake [2]
dpkg-architecture -aarmhf -c make clean [3]

Next the qmake mkspecs are all stored in /usr/share/qt5 which is also
odd, because they are not identical between architectures, thus
ideally all of them should be moved to
/usr/lib/$(DEB_HOST_MULTIARCH)/qt5/mkspec

Hence at the moment, I have to use sed on the generated Makefiles to
replace -lGL with -lGLESv2.

But overall one does able to cross compile qml extensions, even using
qmake. But the co-installability of ubuntu-sdk for multiple
architectures must be resolved, otherwise we will be stuck with using
chroots and jumping between host & chroot to complete a single:
configure-clean-build cycle. [4]

I have started on changing packaging for qtbase-opensource-src but it
seems like it's a rather big task which hopefully SDK team can take
over [5] thus at the moment I'm still monkey-patching in-place system
level directories.

There is README in the [1] branch outlining how I have cross-compile
the stock qmlextension template example generated by the ubuntu-sdk.
And I'm happy to assist & answer any questions about multiarch &
co-installability requirements here. So far I have filed a couple of
bugs which must be resolved to allow cross-compilation for the
ubuntu-sdk [6] There are a few more, e.g. i couldn't install a few
optional qdeclarative plugins due to non-multiarch reverse
dependencies.

[1] lp:~ubuntu-core-dev/+junk/qmake-cross
[2] -a means cross-compile from by build architecute to that target
host architecture, e.g. armhf in this case.
[3] if the generated makefiles have "include
/usr/share/dpkg/architecture.mk" then there will be no need to wrap
calls to make.
[4] well since replacing ones host system GL with something else does
usually break stuff
[5] lp:~xnox/kubuntu-packaging/qtbase-opensource-src
[6] https://bugs.launchpad.net/ubuntu/+bugs?field.tag=qmake-cross