Can't find the 'libpq-fe.h header when trying to install pg gem

Question

I am using the Ruby on Rails 3.1 pre version. I like to use PostgreSQL, but the problem is installing the pg gem. It gives me the following error:

Answer 1

Just for the record:

Ruby on Rails 4 application in OS X with PostgresApp (in this case 0.17.1 version needed - kind of an old project):

gem install pg -v '0.17.1' -- --with-pg-config=/Applications/Postgres.app/Contents/Versions/9.3/bin/pg_config