TT#23584 Honor parallel build support in the overrides

The --parallel option is already the default with debhelper >= 10, but
that does not apply to any overrides we use, which means we were not
honoring parallel builds there.

Change-Id: I3b50098aaf1461d3591989b2779b157562a23c97
(cherry picked from commit c7f3f69472)

debian/rules

@@ -19,6 +19,13 @@ export APP_NAME=ngcp-sems
 export SYSTEM_SAMPLECLOCK_RATE=48000LL
 export USE_THREADPOOL=yes
 
+# Enable parallel builds for overrides.
+NUMJOBS = 1
+ifneq (,$(filter parallel=%,$(DEB_BUILD_OPTIONS)))
+  NUMJOBS = $(patsubst parallel=%,%,$(filter parallel=%,$(DEB_BUILD_OPTIONS)))
+  MAKEFLAGS += -j$(NUMJOBS)
+endif
+
 %:
 	dh $@