Fetch a snapshot build before `make`.

Fixes #14.

I know it is nasty but it works. I am asking gentoo-user ML how to treat
environment variables. When I get the *true* solution current workaround
will be replaced.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #15 from omasanori/fix-issue-14

Fetch a snapshot build before `make`.
Pass an environment variable properly.

The nasty substitution has been removed. ${ECONF_SOURCE} could be empty,
so CFG_SRC_DIR="${ECONF_SOURCE}" might be unset. ${ECONF_SOURCE:-.} is
the *solution*.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>