debian: Set MAKEFLAGS to -jN from parallel=N in DEB_BUILD_OPTIONS

Honor parallel build option.

debian/changelog

@@ -132,6 +132,7 @@ dpkg (1.18.11) UNRELEASED; urgency=medium
     - Stop compressing the dpkg.deb package with gzip.
     - Move dpkg to be the first binary package stanza in debian/control, as
       debhelper assigns special meaning by considering it the main package.
+    - Set MAKEFLAGS to -jN from parallel=N in DEB_BUILD_OPTIONS.
   * Documentation:
     - Update custom changelog parser API support status in README.api.
     - Fix typos in docs and man pages. Thanks to Jakub Wilk <jwilk@debian.org>

debian/rules

@@ -41,6 +41,7 @@ endif
 NUMJOBS = 1
 ifneq (,$(filter parallel=%,$(DEB_BUILD_OPTIONS)))
   NUMJOBS = $(patsubst parallel=%,%,$(filter parallel=%,$(DEB_BUILD_OPTIONS)))
+  MAKEFLAGS += -j$(NUMJOBS)
 endif
 
 D := $(CURDIR)/debian/tmp