Add support for SFCGAL 1.1

[vmo]

Here is a summary of the attached patch.

In doc/reference_sfcgal.xml

• added doc for ST_3DDifference, ST_3DUnion, ST_Volume
• added doc for ST_MakeSolid and ST_IsSolid that are pure postgis functions to set/get the solid flag

In postgis/lwgeom_backend_api.c

• added backend switch for difference and union

In postgis/lwgeom_geos.h and postgis/lwgeom_geos.c

• changed the name of difference and union
• exposed those renamed functions

In postgis/lwgeom_sfcgal.c

• added difference, union, volume
• added makesolid and issolid

In postgis/sfcgal.sql.in

• added ST_3DDifference, ST_3DUnion and ST_Volume
• added ST_MakeSolid and ST_IsSolid

In regress/sfcgal/concave_hull.sql

• fixed test failure caused by insufficient tolerance

In regress/sfcgal/empty.sql, regress/sfcgal/empty_expected, regress/sfcgal/regress_ogc.sql and regress/sfcgal/regress_ogc_expected

• fixed test to avoid failure on equivalent representation of a polygon (different starting point for a ring)

[strk]

Would be nice to get this in PostGIS-2.2. Even better if we're ready for SFCGAL-1.2.0 (yet to be released)

[colivier]

Commited in trunk as r13709.

[robe]

Please see #3182