Using Project/Configure Selection simply runs the selected QMake executable on the project source tree but fails to take any extra arguments into account (say, -spec linux-clang). Running QMake directly via the Project Manager's context menu works correctly AFAICT.
Source code hint: Check kdevelop.git:plugins/qmakemanager -- start by grepping for 'args' or 'extraArguments'. This is probably easy to fix. Look into CMake support to find inspiration how to solve.
Note: Just a hint if someone wants to pick this up; not meant to RJVB directly.
> not meant to RJVB directly. Didn't take it that way (but I'd have figured out a fix if I used QMake projects more often). (And thanks for correcting the subject. Usually I notice when my sh*tty notebook keyboard didn't register a stroke...)
This bug is still present in KDevelop 5.6.1. It looks like both the QMake Builder and QMake Manager plugins have a QMakeJob class in the code, but they differ quite a bit. The latter plugin adds the "Run QMake" context menu entry, which works correctly (see QMakeProjectManager::slotRunQMake where it adds the arguments to the job). It looks like part of that code should be added/moved to QMakeBuilder::configure to fix this.