PostGIS adds support for geographic objects to the PostgreSQL
object-relational database. In effect, PostGIS "spatially
enables" the PostgreSQL server, allowing it to be used as a
backend spatial database for geographic information systems
(GIS).

SFCGAL is an optional dependency for advanced 3D support. 
These advanced functions are not included in the postgis 
extension, but they can be make available by loading 
/usr/share/postgresql/contrib/postgis-2.1/sfcgal.sql.

To disable the GTK-based graphical interface for shp2pgsql,
set GUI=no.