qt: pass debug mode (if enabled) to qmake

The behavior may be different when qmake generates the build files depending on the build mode.

For example, with MinGW-w64 -Wl,-s (which stands for stripping symbol information) is added to the linker flags if build mode is (directed or assumed to be) release.

If debug mode is not enabled, the default behavior is retained, which appears to be release. I specifically did not request release mode when debug is false, because debug being false does not necessarily mean release.

Request review @robUx4.