Add post-check phase to PostGIS build

[robe]

Right now we are probalby the only extension that does our check before install. This is great because it allows you to do a quick check before you screw up your postgresql system, but is bad because there is no way verify the install actually works and will install all the necessary plumbing. It also means we have no way of verifying CREATE EXTENSION or even doing a true check of upgrade.

I propose a post-check option to solve this that is a check done after install. Since it is done after install it has a couple of benefits

1. It can be run with CREATE EXTENSION thus testing our extension building

2. We can easily squeeze in a true upgrade test in here since our model allows for installing multiple versions of postgis in the same instance.

So a switch something like —with-upgrade-from=2.0.4SVN

Will in post-check phase do a:

CREATE EXTENSION postgis VERSION "2.0.4SVN";
ALTER EXTENSION postgis UPDATE TO "2.1.0";

Since both winnie and debbie are doing make installs (have to double-check on debbie, but winnie definitely is), they'll be able to take advantage of this new flag to do a true upgrade regression test.