Overview
Attempting to install the Ruby gem “pg” or other PostgreSQL-dependent gems on v6+ platforms fail with a similar sample response:
Can't find the PostgreSQL client library (libpq) *** extconf.rb failed *** Could not create Makefile due to some reason, probably lack of necessary libraries and/or headers. Check the mkmf.log file for more details. You may need configuration options. Provided configuration options: --with-opt-dir
Cause
PostgreSQL libraries are located in a separate directory, /usr/pgsql-XX, where XX is the version number on these platforms. This path is not picked up in normal configuration.
Solution
Install the gem manually while specifying a path to pg_config, which is located in /usr/pgsql-XX/bin. For example, on Sol, which ships with PostgreSQL 9.3, the correct command to install pg is as follows:
gem install pg -v '0.18.3' -- --with-pg-config=/usr/pgsql-9.3/bin/pg_config