PGDG strech : postgresql-11-postgis-2.5 won't install beacause of missing dependency

Hello.

I try to install postgis 2.5 from stretch-pgdg repo on an otherwise dist-upgraded, fresh Debian 9.

libgeos-c1v5 is not found

```
root(at)db-01:~# cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 9 (stretch)"
NAME="Debian GNU/Linux"
VERSION_ID="9"
VERSION="9 (stretch)"
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"

root(at)db-01:~# apt install postgresql-11-postgis-2.5
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
postgresql-11-postgis-2.5 : Depends: libgeos-c1v5 (>= 3.7.0) but 3.5.1-3 is to be installed
E: Unable to correct problems, you have held broken packages.

root(at)db-01:~# apt policy postgresql-11-postgis-2.5 libgeos-c1v5
postgresql-11-postgis-2.5:
Installed: (none)
Candidate: 2.5.3+dfsg-2.pgdg90+1
Version table:
2.5.3+dfsg-2.pgdg90+1 500
500 http://apt.postgresql.org/pub/repos/apt stretch-pgdg/main amd64 Packages
libgeos-c1v5:
Installed: (none)
Candidate: 3.5.1-3
Version table:
3.5.1-3 500
500 http://deb.debian.org/debian stretch/main amd64 Packages
```

stretch-backports repo is active on this host, too.

---
Bruno Dupuis
http://www.kosc-telecom.fr