Revert "Remove NPROCS from Makefile, make is able to automatically detect the optimal number of parallel threads"

Problem here is; This process requires alot of memory. If host memory is too small it will segfault somewhere inside AMD's OpenCL runtime library. Therefore it's better to stick to NPROCS as part of a workaround.

This reverts commit e5adccbf38.

src/Makefile

@@ -4,16 +4,17 @@
 ##
 
 ##
-## Detect host OS
+## Detect number of processors
 ##
 
+NPROCS   := $(shell grep -c ^processor /proc/cpuinfo)
 OS       := $(shell uname)
 
 ##
 ## Makefile flags
 ##
 
-MAKEFLAGS += -l -j -rR --no-print-directory
+MAKEFLAGS += -l -j $(NPROCS) -rR --no-print-directory
 
 ifneq ($(findstring clean,$(MAKECMDGOALS)),)
 MAKEFLAGS += -j 1