Added support for INSTALL_PREFIX in QMake project

[dpurgin]

Hi,

we're using the keyboard in a Buildroot environment which executes make install on the packages to install them to the target rootfs.

The current .pro tries to install the keyboard into the toolchain's sysroot, and it is not desired.

The patch provides a new configuration variable INSTALL_PREFIX to override the installation path. If it is not specified, the installation is performed into the Qt installation directory as previously.

Example:

$ qmake CONFIG+=release INSTALL_PREFIX=/home/build/rootfs_custom
Project MESSAGE: This project is using private headers and will therefore be tied to this specific Qt module build version.
...
$ make 
...
$ make install
/opt/buildroot-imx8m/host/bin/qmake -install qinstall -exe libcutekeyboardplugin.so /home/build/rootfs_custom/plugins/platforminputcontexts/libcutekeyboardplugin.so
/opt/buildroot-imx8m/host/bin/qmake -install qinstall /home/build/workspace/cutekeyboard/src/qml/qmldir /home/build/rootfs_custom/qml/QtQuick/CuteKeyboard/qmldir
/opt/buildroot-imx8m/host/bin/qmake -install qinstall /home/build/workspace/cutekeyboard/src/qml/AlternativeKeysPopup.qml /home/build/rootfs_custom/qml/QtQuick/CuteKeyboard/AlternativeKeysPopup.qml
...

Cheers

[AndreaRicchi]

Nice! Thanks