liblttng-ust dependency makes lomiri-ui-toolkit difficult to build and seems like it shouldn't be mandatory

It seems like using qmake, installing a library to e.g. /usr/local/lib would usually work via qmake PREFIX=/usr/local.

However, lomiri UI toolkit's make install after running qmake seems to just ignore qmake's PREFIX option. To install it with non-standard prefixes which seems to e.g. b required for a flatpak build, it seems like it would be required that 1. either this option was fixed to work like in Qt, or 2. that lomiri UI toolkit's README.md specifies an alternate option with the same outcome.

It seems like my final install part is failing due to the liblttng-ust dependency not working. Perhaps this dependency shouldn't be mandatory, since it seems to concern debugging the toolkit.

My apologies if I missed some obvious way to do this right. I tested this with the latest commit.