OK, first of all, you need the curl-7.19.4-1.src.rpm as that's the one containing the sources. The one you downloaded contains the already compiled version.
Second, before you start breaking systems with malformed packages, get comfortable with the build-process of the software first. Get a feel for the various configure switches, the build process itself, ...
Then, and only then, apply that knowledge to the spec file for rpmbuild.
OR you can use the source RPM from CentOS (
linky), a 100% RHEL compatible distribution.