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/ggsn_tests/gen_links.sh b/ggsn_tests/gen_links.sh
index 6dd2d62..8098f9b 100755
--- a/ggsn_tests/gen_links.sh
+++ b/ggsn_tests/gen_links.sh
@@ -1,6 +1,6 @@
 #!/bin/sh
 
-BASEDIR=~/projects/git
+BASEDIR=../deps
 
 gen_links() {
 	DIR=$1