Building rustc requires a working rustc. Solaris is a tier 3 platform, so rust-lang.org doesn’t distribute binaries (see https://doc.rust-lang.org/book/getting-started.html). Therefore, you either need to cross-compile it or find an unofficial binary. (Not sure where to find unofficial binaries for Solaris in particular; maybe @nbaksalyar knows?)
You can pass “–enable-local-rust” to configure to prevent it from trying to download a binary from rust-lang.org, but obviously that only works if you already have rustc installed.