Make uninstall does not work for custom DESTDIR / PREFIX

[karlch]

Running make uninstall after installing with a custom DESTDIR or PREFIX does not work as the install_log.txt does not capture these. Should be fixed by either:

• Writing the absolute path to install_log.txt
• Updating the uninstall to prefix DESTDIR and PREFIX. Not preferred, as one would have to manually remember what was used at-the-time.