Emulate DESTDIR behavior for per-component PPA deployment

[scheibel]

This enables the combination of the -DCMAKE_INSTALL_COMPONENT variable with the DESTDIR convention of GNU make install (https://www.gnu.org/prep/standards/html_node/DESTDIR.html).

The CMake OPTION_DEPLOY_DESTDIR option (actually, a cached string variable) is added for Linux systems that takes the value of the DESTDIR parameter/macro/environment variable and generates one target per component (destdir-deploy-${component}) that calls the preinstall target and later executes the install with the DESTDIR construct and the current component.

With this extension it is possible to deploy only parts of the CMake components for a debian source package. A typical workflow would be:

> cd build
build > cmake .. -DOPTION_DEPLOY_DESTDIR=debian/tmp
build > make destdir-deploy-runtime
build > cd ..
> dpkg-gencontrol -pcmake-init
> dpkg --build debian/tmp

[scheibel]

Closed in favor of a more elegant solution.