#2794 sfcgal regress sfcgal doesn't install if RUNTESTFLAGS is set by user because trying to reuse that flag.  Define new flag SFCGALTESTFLAGS and pass that along.

git-svn-id: http://svn.osgeo.org/postgis/trunk@12703 b70326c6-7e19-0410-871a-916f4a2858ee