Using $(MAKE) variable instead of hardcoding parameters.

That way, that make-invocation will inherit parameters given on the
command-line, e.g. the -j job parameter.

Makefile

diff --git a/Makefile b/Makefile
index 318d1a9..2643191 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,8 @@
 	mkdir -p build/tests;
 	mkdir -p build/dist;
 	mkdir -p dist;
-	cd build; cmake ../ -DCMAKE_BUILD_TYPE=Release; make -j 4 
+	cd build; cmake ../ -DCMAKE_BUILD_TYPE=Release
+	$(MAKE) -C build
 	cd build; ../tests/regression.tcl mt=0 show_diff
 
 test:
@@ -15,4 +16,3 @@
 
 install:
 	cd build; make install
-