Do not allow installing postgis twice in the same database (#2580)

Checks for the presence of a "postgis_version" function,
and if found raises an exception with a message with
the schema containing it.

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12164 b70326c6-7e19-0410-871a-916f4a2858ee