Add SFCGAL support cf #2254. Include SFCGAL support, postgis backend handling (GEOS/SFCGAL). Regress tests, documentation prototypes and also additional force_sfs function.

git-svn-id: http://svn.osgeo.org/postgis/trunk@11389 b70326c6-7e19-0410-871a-916f4a2858ee