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.