testing: Add option for a quick rebuild of strongSwan

This shaves off about 1 minute of build time on my machine.  We also
don't need the separate build step and can just run `make install`.

testing/scripts/recipes/013_strongswan.mk

@@ -127,11 +127,8 @@ $(PKG): $(TAR)
 	echo "$(SWANVERSION)" > /root/shared/.strongswan-version
 
 configure: $(BUILDDIR)
-	cd $(BUILDDIR) && $(DIR)/configure $(CONFIG_OPTS)
+	[ -n "$(QUICK_REBUILD)" ] || (cd $(BUILDDIR) && $(DIR)/configure $(CONFIG_OPTS))
 
-build: configure
-	cd $(BUILDDIR) && make -j $(NUM_CPUS)
-
-install: build
+install: configure
 	cd $(BUILDDIR) && make -j install && \
 		cd $(DIR)/src/libcharon/plugins/vici/python && python3 setup.py install