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.