packaging: Limit the number of parallel jobs when building Qt.

Workaround <https://bugreports.qt.io/browse/QTBUG-94800>.

This is to avoid out of memory conditions during the build, which
requires up to about 2 GiB of memory per parallel process.  There was
also a problem with how the MAKEFLAGS was passed, which led
qtwebengine to be built using all the CPU cores even when specifying a
lesser NO_CPUS value.

* packaging/rules/debian-qt/rules (min, max): New functions.
(NO_CPUS): Express in terms of max.
(AVAILABLE_MEMORY, MEMORY_REQUIRED_PER_CORE, COMPUTED_JOB_COUNT)
(JOB_COUNT, MAX_PARALLEL_BUILDS): New variables.
(override_dh_auto_build): Override to manually control job count.
Specify NINJAFLAGS for the Chromium build.
* packaging/rules/rpm/jami-libqt.spec (min, max, cpu_count)
(available_memory, computed_job_count, job_count): New macros.
(%build): Re-indent and use the above 'job_count' variable as
the number of parallel jobs.
(%install): Likewise.

Change-Id: I9134c1e63e92104a1da8435670e2919ce692040c