Build On-screen keyboard "onboard" for BLFS 7.5

On-screen keyboard "onboard" requires Python 3. Also, we have to install the following packages:

setuptool
---------
python3 setup.py install

python-distutils-extra
-----------------------
python3 setup.py install

hunspell
--------
./configure --prefix=/usr --disable-static --sysconfdir=/etc
make
make install

onboard
-------
./setyp.py install
glib-compile-schemas /usr/share/glib-2.0/schemas

gtk-update-icon-cache /usr/share/icons/hicolor