Vivian,
It would be much easier for you to install the PostgreSQL 7.1 RPM packages instead of trying to compile and install it from sources. It is not a trivial task. The RPMs are available from PostgreSQL's web site, or from an older Red Hat version CD.
If you have your mind set on compiling, you could follow the PostgreSQL documentation on how to compile and install. It describes de needed dependencies and steps.
-Roberto
