Add parallel build support to 0ad/debian/rules.

diff --git a/debian/rules b/debian/rules
index 9415753..6a0fd19 100755
--- a/debian/rules
+++ b/debian/rules
@@ -1,9 +1,14 @@
 #!/usr/bin/make -f

-ifeq ($(findstring parallel=,$(DEB_BUILD_OPTIONS)),)
-   export DEB_BUILD_OPTIONS+=parallel=$(shell getconf _NPROCESSORS_ONLN)
+DEB_BUILD_OPTIONS_PARALLEL=$(filter parallel=%,$(DEB_BUILD_OPTIONS))
+fneq (,$(DEB_BUILD_OPTIONS_PARALLEL))
+    NUMJOBS = $(patsubst parallel=%,%,$(DEB_BUILD_OPTIONS_PARALLEL))
+else
+    NUMJOBS = $(shell getconf _NPROCESSORS_ONLN)
 endif

+MAKEFLAGS += -j$(NUMJOBS)
+
 include /usr/share/dpkg/architecture.mk

 export CBUILD=$(DEB_BUILD_GNU_TYPE)
@@ -24,6 +29,7 @@ override_dh_auto_clean:

 override_dh_auto_build:
    build/workspaces/update-workspaces.sh \
+       -j$(NUMJOBS) \
        --bindir=/usr/games \
        --libdir=/usr/lib/games/0ad \
        --datadir=/usr/share/games/0ad \