Add check for PostgreSQL version compatibility in postgis_full_version

Adds postgis_scripts_pgsql_version() and postgis_pgsql_version()
internal functions, each returning an encoded version number,
and has postgis_full_version() compare running vs. build-time used

Fixes #3822 (untested)

git-svn-id: http://svn.osgeo.org/postgis/trunk@15602 b70326c6-7e19-0410-871a-916f4a2858ee