QMake ignores my `PREFIX`/`INSTALL_PREFIX`/etc. variables!

This article was originally published at https://gist.github.com/joepie91/b0041188c043259e6e1059d026eff301 .  
 QMake does not have a standardized configuration variable for installation prefixes -  PREFIX and INSTALL_PREFIX only work if the project files for the software you're building specify it explicitly. 
 If the project files have a hardcoded path, there's still a workaround to install it in $out anyway, without source code or project file patches: 
 {
 # ...
 preInstall = "export INSTALL_ROOT=$out";
 # ...
} 
 This INSTALL_ROOT environment variable will be picked up and used by make install , regardless of the paths specified by QMake.