Adding support for parallel recursive compilation for the tests

Makefile.am

@@ -4,7 +4,7 @@ SUBDIRS = lib benchmarks tests
 .PHONY: tests
 
 tests:
-	make -C tests tests
+	$(MAKE) -C tests tests
 
 AM_CXXFLAGS += -I$(top_builddir)/include
 ACLOCAL_AMFLAGS = -I m4

tests/Makefile.am

@@ -9,4 +9,4 @@ endif
 include Make.inc
 
 subtests:
-	for d in $(SUBDIRS); do make -C $${d} tests; done
+	for d in $(SUBDIRS); do $(MAKE) -C $${d} tests; done