Force rtpostgis.so rebuild on librtcore.a change

git-svn-id: http://svn.osgeo.org/postgis/trunk@12779 b70326c6-7e19-0410-871a-916f4a2858ee
Add note about extent ratainment in ST_Resize, cross link docs

Closes #2833

git-svn-id: http://svn.osgeo.org/postgis/trunk@12780 b70326c6-7e19-0410-871a-916f4a2858ee