pg_config executable not found

I am having trouble installing psycopg2. I get the following error when I try to `pip install psycopg2`: ``` Error: pg_config executable not found. Please add the directory containing pg_config to the PATH or specify the full executable path with the option: python setup.py build_ext --pg-config /path/to/pg_config build ... or with the pg_config option in 'setup.cfg'. ---------------------------------------- Command python setup.py egg_info failed with error code 1 in /tmp/pip-build/psycopg2 ``` But the problem is `pg_config` is actually in my `PATH`; it runs without any problem: ``` $ which pg_config /usr/pgsql-9.1/bin/pg_config ``` I tried adding the pg_config path to the `setup.cfg` file and building it using the source files I downloaded from their website (http://initd.org/psycopg/) and I get the following error message! ``` Error: Unable to find 'pg_config' file in '/usr/pgsql-9.1/bin/' ``` But it is actually THERE!!! I am baffled by these errors. Can anyone help please? By the way, I `sudo` all the commands. Also I am on RHEL 5.5.