reenable automatic parallel build of APT

It works for a while now in manual tests, now lets see how it will
perform if enabled for all by default automatically.

buildlib/defaults.mak

@@ -172,12 +172,11 @@ ifeq ($(NUM_PROCS),1)
   PARALLEL_RUN=no
 endif
 
-# mvo: commented out, lead to build failures in the arch-build target
-#ifndef PARALLEL_RUN
-# PARALLEL_RUN=yes
-# .EXPORT: PARALLEL_RUN
-# # handle recursion
-# ifneq ($(NUM_PROCS),)
-#  MAKEFLAGS += -j $(NUM_PROCS)
-# endif
-#endif
+ifndef PARALLEL_RUN
+ PARALLEL_RUN=yes
+ export PARALLEL_RUN
+ # handle recursion
+ ifneq ($(NUM_PROCS),)
+  MAKEFLAGS += -j $(NUM_PROCS)
+ endif
+endif