### failure to compile cython .pyx when cimporting some function from sage

Hello,

I am working on a Sage graph project, and for optimization,
I have chosen to move it to Cython.
The code compiles just fine as is, but for further optimization
I will need to use a union find data structure in my `cdef`

function.
So far, I was using `DisjointSet`

from `sage.sets.disjoint_set`

in a separate function: my issue is I do not know
how to `cdef`

such an object.

My initial attempt to solve this was to `cimport`

the disjoint set class from the `sage/sets/disjoint_set.pyx`

file,
and use it to `cdef`

.

This would in summary be something like this :

```
...
from sage.sets.disjoint_set cimport
```~~DisjointSet
~~DisjointSet_of_hashables
cdef ~~DisjointSet ~~DisjointSet_of_hashables D
D = DisjointSet(10)
...

Though this does not compile : I get an error related to a library
(that I assumed was included in the install of Sage).
Just in case it was not, I manually installed GAP
(which did not solve the problem).

```
/usr/bin/ld : ne peut trouver -lgap
collect2: error: ld returned 1 exit status
error: command 'x86_64-linux-gnu-g++' failed with exit status 1
```

This message is partly in French but it translates exactly to

```
/usr/bin/ld: cannot find -lgap
```

I believe this is mostly likely due to me misunderstanding
how to import a class from the Sage Cython files.
I had a similar problem when trying to work with bitsets
(although the error was on the library PARI instead of GAP).

In case it is relevant, I am working on Ubuntu 20.04,
installed Sage with apt (Sage version is 9.0).

Thank you in advance for any help.

Pierre