Fix main Makefile to allow parallel builds

Modify the main Makefile so that post_build target is not started in
parallel with tests and programs recipe.
diff --git a/Makefile b/Makefile
index 39fc342..d2fa6b8 100644
--- a/Makefile
+++ b/Makefile
@@ -6,7 +6,8 @@
 
 .PHONY: all no_test programs lib tests install uninstall clean test check covtest lcov apidoc apidoc_clean
 
-all: programs tests post_build
+all: programs tests
+	$(MAKE) post_build
 
 no_test: programs