Missing dependnecy in sfcgal 1.3.7 on focal

sfcgal(libsfcgal1, libsfcgal-dev) v1.3.7 on focal (Ubuntu 20.04) causes a link error when used.

```
/usr/bin/ld: /lib/x86_64-linux-gnu/libSFCGAL.so.1: undefined reference to `operator<<(std::ostream&, __mpz_struct const*)'
/usr/bin/ld: /lib/x86_64-linux-gnu/libSFCGAL.so.1: undefined reference to `operator>>(std::istream&, __mpz_struct*)'
```

investigation
-------------

Here is output of ldd
miurahr@miurahr-ThinkPad-T14s-Gen-1:~/Projects/Geo/sfcgal-1.3.7/debian/libsfcgal1/usr/lib/x86_64-linux-gnu$ ldd libSFCGAL.so.1
 linux-vdso.so.1 (0x00007ffdeebf3000)
 libboost_serialization.so.1.71.0 => /usr/lib/x86_64-linux-gnu/libboost_serialization.so.1.71.0 (0x00007f162c238000)
 libmpfr.so.6 => /usr/lib/x86_64-linux-gnu/libmpfr.so.6 (0x00007f162c1b7000)
 libgmp.so.10 => /usr/lib/x86_64-linux-gnu/libgmp.so.10 (0x00007f162c133000)
 libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f162bf51000)
 libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f162be02000)
 libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f162bde7000)
 libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f162bbf3000)
 /lib64/ld-linux-x86-64.so.2 (0x00007f162cc45000)
 libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f162bbd0000)

I found there is not a libgmpxx dependency.

details
-------

Upstream sfcgal 1.3.7 recommend dependency CGAL 4.3 and disables GMPXX.
Because Ubuntu distribute CGAL 5.0, so sfcgal source has been patched to link with CGAL 5.0 and try to enable GMPXX, but it is incomplete.

This is an upstream change to support CGAL 5.0.

https://gitlab.com/Oslandia/SFCGAL/-/commit/6caae06aff00afc2c60921dcea6cbc4cd2af2405

1. change to CMakeLists.txt to 'find_package(CGAL)' from 'find_package(CGAL 4.3)'
2. add to CMakeLists.txt a definition 'add_definitions( "-DCGAL_USE_GMPXX=1" )'
3. add to src/CMakeLists.txt a dependency 'target_link_libraries( SFCGAL gmpxx )'
4. remove config.h definition '#define CGAL_DO_NOT_USE_GMPXX 1'

 Ubuntu package does No.1 and No.2, but unfortunately did not No.3 and No.4.

Missing No.3 cause the link error.

I attached a patch to intend to add to 'debian/patches/'

Other consideration
-------------------

debian's sfcgal is not v1.3.7. this is only for the Ubuntu and its derivatives issue.

Affected project
----------------

GDAL is a project to link with SFCGAL and cmake build script got link error with SFCGAL with the bug.

https://github.com/miurahr/cmake4gdal/issues/13