Fix BUILD_VERSION value in non-repo, non-development environment.

When the tarball is unpacked, value set in BUILD_VERSION should default to
empty string. Previously, it was setting the value to literal 0, which was appropriate
in the SVN world.

This also updates the testRelease script to verify the version number based on what
is in version_num.h.

Open question: what should happen when `CHPL_DEVELOPER` is true, and
therefore `developer` is true in version.cpp, and the release tarball is built?

In the old SVN world, the version number would be `1.9.0.0` (for example). With this
change, as-is right now, the version number is `1.10.0.` (note the trailing period that
occurs because BUILD_VERSION has the empty string). A couple ways to address
this are: 1) make the default in BUILD_VERSION `"0"` and check for that in
version.cpp, and 2) get rid of the developer check in version.cpp and just check that
BUILD_VERSION does not equal empty string.
I'd say that, for consistency of behavior with what came before (and to avoid the stray period), let's echo "0" here and test against "0" in developer mode.  I wouldn't be completely opposed to removing the || developer test in version.cpp either, but think that there may be benefit in retaining that behavior.