Support optional debian parallel builds

With this commit we allow getting `make -jN` behavior by setting
`parallel=N` in `DEB_BUILD_OPTIONS`.  Setting `parallel=` will result
in `make -j` being run (i.e. no limit on the number of jobs).  The
default is to run one job at a time; this is still the safest bet when
building all modules.

Thanks to Peter Wu for suggesting a different way to do this.

ref: https://www.debian.org/doc/debian-policy/ch-source.html#s-debianrules-options

debian/rules

@@ -20,6 +20,11 @@ export CFLAGS:=$(FS_CFLAGS)
 export CXXFLAGS:=$(FS_CXXFLAGS)
 export LDFLAGS:=$(FS_LDFLAGS)
 
+NJOBS=1
+ifneq (,$(filter parallel=%,$(DEB_BUILD_OPTIONS)))
+  NJOBS=$(patsubst parallel=%,%,$(filter parallel=%,$(DEB_BUILD_OPTIONS)))
+endif
+
 show_vars= \
 	echo; \
 	echo "Making target $@"; \
@@ -91,7 +96,7 @@ override_dh_auto_configure: .stamp-configure
 
 .stamp-build: .stamp-configure
 	@$(call show_vars)
-	make
+	make -j$(NJOBS)
 	touch $@
 
 override_dh_auto_build: .stamp-build