/usr/src/2.4.19? You're sure on that directory name? I guess it works, but it isn't what they're usually called. Oh well, shouldn't be a huge deal.
Try this out:
ln -s /usr/src/2.4.19 build
Then blow away the directory that the NVIDIA_kernel tarball created, and re-decompress it (just a make clean may work, but blowing it away makes absolutely sure it's completely clean). Then do a make install again, still as root.
As long as /lib/modules/2.4.19/build is pointing at the root of the correct, configured (and preferably built, but that's not strictly necessary) kernel source tree, there's no reason you'd need to use SYSINCLUDE.
If you get errors, post them.