Does not respect PREFIX

[Michał Sawicz]

Steps:
> apt install devscripts equivs
> bzr branch lp:ubuntu-ui-toolkit
> mkdir uitk-install
> mk-build-deps -s sudo -i ubuntu-ui-toolkit
> cd ubuntu-ui-toolkit
> qmake PREFIX=`readlink -f ../uitk-install`
> make install

Expected:
* builds and installs fine

Current:
* errors out:
> make[3]: Entering directory '/home/michal/dev/canonical/unity8/uitk-amd64/modules/Ubuntu/Components'
> ALARM_BACKEND=memory /usr/lib/x86_64-linux-gnu/qt5/bin/qmlplugindump -notrelocatable Ubuntu.Components 0.1 ../../ 2>/dev/null > //usr/lib/x86_64-linux-gnu/qt5/qml/Ubuntu/Components/plugins.qmltypes
> Makefile.ComponentModule:295: recipe for target 'install_plugins_qmltypes' failed
> make[3]: *** [install_plugins_qmltypes] Error 2
> make[3]: Leaving directory '/home/michal/dev/canonical/unity8/uitk-amd64/modules/Ubuntu/Components'
> Makefile:81: recipe for target 'sub-ComponentModule-pro-install_subtargets' failed

When cross-building:
> make[3]: Entering directory '/home/michal/dev/canonical/unity8/ubuntu-ui-toolkit/modules/Ubuntu/Components/ListItems'
> mkdir: cannot create directory ‘/usr/lib/arm-linux-gnueabihf/qt5/qml/Ubuntu/Components’: Permission denied
> Makefile:267: recipe for target 'install_qml_files_f4eaca92-c1f0-47bd-bdd3-eda299079fc0' failed
> make[3]: *** [install_qml_files_f4eaca92-c1f0-47bd-bdd3-eda299079fc0] Error 1
> make[3]: Leaving directory '/home/michal/dev/canonical/unity8/ubuntu-ui-toolkit/modules/Ubuntu/Components/ListItems'
> Makefile:62: recipe for target 'sub-ListItems-install_subtargets' failed

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: qtdeclarative5-ubuntu-ui-toolkit-plugin 1.2.1433+15.04.20150306-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-9.9-generic 3.19.1
Uname: Linux 3.19.0-9-generic x86_64
ApportVersion: 2.16.2-0ubuntu3
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Mar 17 21:17:42 2015
SourcePackage: ubuntu-ui-toolkit
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)