Install libpq-dev package error

This should let you install the package you need :) Just tried it myself, I am not sure you will need all the packages on the second line, try it and see :)

apt-get update
apt-get install libxslt-dev libxml2-dev libpam-dev libedit-dev
aptitude install -t squeeze-backports postgresql-server-dev-9.1

You must setup postgresql-server-dev-X.Y, where X.Y. your's servers version, and it will install libpq-dev and other servers variables at modules for server side developing. In my case it was

apt-get install postgresql-server-dev-9.5

Reading package lists... Done Building dependency tree Reading state information... Done The following packages were automatically installed and are no longer required: libmysqlclient18 mysql-common Use 'apt-get autoremove' to remove them. The following extra packages will be installed:
libpq-dev Suggested packages: postgresql-doc-10 The following NEW packages will be installed: libpq-dev postgresql-server-dev-9.5

I have a similar problem, I solved it by installing a specific version of libpg5

sudo apt install libpq5=10.12-0ubuntu0.18.04.1

My machine is Ubuntu 18.04