Makefile: clone dependencies into 'deps' folder

We used to rely on out-of-tree git clones to be prepared by the user.

This commit changes the system to make sure we clone all git repos
we depend upon into the 'deps' folder, and then setup the symlinks
to that folder.  As a result, we should be able to support
self-contained builds using the makefiles in this repo.
diff --git a/Makefile b/Makefile
index e017129..6a470a0 100644
--- a/Makefile
+++ b/Makefile
@@ -14,7 +14,11 @@
 #	make bsc/all
 #	make bsc	(equivalent to bsc/all)
 
-default: all
+default: deps all
+
+.PHONY: deps
+deps:
+	$(MAKE) -C deps
 
 compile: $(foreach dir,$(SUBDIRS),$(dir)/compile)
 clean: $(foreach dir,$(SUBDIRS),$(dir)/clean)