add a flag for SFCGAL function cf #2317. Remove useless ST_MakeSolid doc entry

git-svn-id: http://svn.osgeo.org/postgis/trunk@11407 b70326c6-7e19-0410-871a-916f4a2858ee
Use ST_MinkowskiSum rather than ST_Minkowski userland.

git-svn-id: http://svn.osgeo.org/postgis/trunk@11408 b70326c6-7e19-0410-871a-916f4a2858ee