I believe your earlier version already was installing to /usr/local. You've just made it explicit. Most configuration scripts accept a --prefix option, but default to /usr/local if you omit the option. I'm pretty sure that Ruby works that way, and it's exceedingly common. (Not that the explicitness isn't a good idea. You don't want to rely on a default without being sure.)

Follow-up: partial output of ./configure --help in a Ruby source directory:

> By default, `make install' will install all the files in `/usr/local/bin', `/usr/local/lib' etc. You can specify an installation prefix other than `/usr/local' using `--prefix', for instance `--prefix=$HOME'.