Enable parallel builds.

configure

@@ -61,15 +61,15 @@ echo ""
 
 cat > Makefile << EOF
 all:
-	@make --no-print-directory -C build-$build/
+	@+make --no-print-directory -C build-$build/
 install:
-	@make install --no-print-directory -C build-$build/
+	@+make install --no-print-directory -C build-$build/
 package:
-	@make package --no-print-directory -C build-$build/
+	@+make package --no-print-directory -C build-$build/
 test:
-	@make test --no-print-directory -C build-$build/
+	@+make test --no-print-directory -C build-$build/
 clean:
-	@make clean --no-print-directory -C build-$build/
+	@+make clean --no-print-directory -C build-$build/
 distclean:
 	@rm -Rf build-$build/
 EOF