Well, that's not a fix as it won't make it work on RHEL5, that's just making the failure message more informative.
Good thing IMHO. configure.ac is supposed to test for minimum features, after all.
I guess I can try compiling static builds against newer curl, but I don't really understand if anyone is interested in RHEL5 builds other than me.
The problem with this idea is that network-using static libraries are not supported by glibc.
You should be able to build your own version of "new" libcurl and compile+link against that.
Adding a --with-libcurl=/usr/local/src/newcurl option to configure is reasonably simple.