debian/rules: Only do parallel build if specified in DEB_BUILD_OPTIONS

It was not nice to use 2 * number of cores in all cases.

Thanks: Jakub Wilk for the suggestion

debian/rules

@@ -9,6 +9,7 @@ export NOISY=1
 export DEB_HOST_GNU_TYPE  ?= $(shell dpkg-architecture -qDEB_HOST_GNU_TYPE)
 export DEB_BUILD_GNU_TYPE ?= $(shell dpkg-architecture -qDEB_BUILD_GNU_TYPE)
 export DEB_HOST_MULTIARCH ?= $(shell dpkg-architecture -qDEB_HOST_MULTIARCH)
+export PARALLEL = $(or $(patsubst parallel=%,%,$(filter parallel=%,$(DEB_BUILD_OPTIONS))),1)
 
 # FOR AUTOCONF 2.52 AND NEWER ONLY
 ifeq ($(DEB_BUILD_GNU_TYPE), $(DEB_HOST_GNU_TYPE))
@@ -17,6 +18,9 @@ else
   confflags += --build $(DEB_BUILD_GNU_TYPE) --host $(DEB_HOST_GNU_TYPE)
 endif
 
+# Set the number of procs to be run in parallel from the options.
+confflags += --with-procs=$(PARALLEL)
+
 # See below
 -include build/environment.mak