DESTDIR not honored for Python

Apart from #380 there is another issue preventing packaging with python, at least in the normal way.

On debian installation will be run with make -j2 install DESTDIR=/<<PKGBUILDDIR>>/debian/tmp. Unfortunately, DESTDIR is ignored for python and it still uses the value of variable CMAKE_INSTALL_PREFIX.

NOte that cmake is called like this: DEB_PYTHON_INSTALL_LAYOUT=deb cmake -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=None -DCMAKE_INSTALL_SYSCONFDIR=/etc -DCMAKE_INSTALL_LOCALSTATEDIR=/var ... to make sure the paths are correct in the package.