Makefile: Allow for overriding job threads for making 'qemu' dir (#762)

Providing a default level of parallelism is perfectly fine, but it
should also be possible to set higher or lower levels of parallelism
easily. This is particularly beneficial for people packaging the
software for Linux distributions.

Makefile

@@ -9,6 +9,13 @@ include pkgconfig.mk	# package version
 
 LIBNAME = unicorn
 UNAME_S := $(shell uname -s)
+# SMP_MFLAGS is used for controlling the amount of parallelism used
+# in external 'make' invocations. If the user doesn't override it, it
+# does "-j4". That is, it uses 4 job threads. If you want to use more or less,
+# pass in a different -jX, with X being the number of threads.
+# For example, to completely disable parallel building, pass "-j1".
+# If you want to use 16 job threads, use "-j16".
+SMP_MFLAGS := -j4
 
 GENOBJ = $(shell find qemu/$(1) -name "*.o" 2>/dev/null) 
 
@@ -204,7 +211,7 @@ qemu/config-host.h-timestamp:
 	cd qemu && \
 	./configure --cc="${CC}" --extra-cflags="$(UNICORN_CFLAGS)" --target-list="$(UNICORN_TARGETS)" ${UNICORN_QEMU_FLAGS}
 	printf "$(UNICORN_ARCHS)" > config.log
-	$(MAKE) -C qemu -j 4
+	$(MAKE) -C qemu $(SMP_MFLAGS)
 	$(eval UC_TARGET_OBJ += $$(wildcard qemu/util/*.o) $$(wildcard qemu/*.o) $$(wildcard qemu/qom/*.o) $$(wildcard qemu/hw/core/*.o) $$(wildcard qemu/qapi/*.o) $$(wildcard qemu/qobject/*.o))
 
 unicorn: $(LIBRARY) $(ARCHIVE)