Need a flag on the sfcgal functions

[robe]

I notice for this function for example:

​http://postgis.net/docs/manual-dev/ST_MakeSolid.html

There is no note saying requires sfcgal installation. I know we have it in its own section so its somewhat obvious except when you aren't coming from table of contents.

I'm also going to need it for filtering functions.

I think we'll need two flags

an: sfcgal_enhanced (for those that overlap)

as well as an sfcgal_required

[colivier]

Have been done as r11407