Fix SFCGAL with CGAL 5
Created by: lbartoletti
I added a condition to link sfcgal with gmpxx since it is necessary when cgal version >= 5.
I tested locally with postgis. As gmpxx is in sfcgal, there is no need to add gmpxx in sfcgal_config.in
Tests:
- Ubuntu and CGAL 5.0 (CI Travis)
- FreeBSD, CGAL 4.13 (local and CI Cirrus)
- FreeBSD, CGAL 5.0.2 (local)
Fix #198 (closed)