I should have probably mentioned the reason I'm asking this
I needed to update the live ebuild of SMPlayer in the Gentoo multimedia overlay. (The live ebuild is installing SMPlayer from Git.) As with other package formats, the dependencies must be correct. mplayer has a configure option for this (--{enable,disable}-freebidi). I assume SMPlayer now needs this, so it should always be forced to enabled in the package? The package has this disabled by default and users (or other packages that depend on mplayer) who need it, can then enable it though a USE flag.
mplayer2 doesn't have a freebidi option, and at runtime it reports that it doesn't recognize the "--noflip-hebrew" option. However, this turned out to be some weird problem with SMPlayer not recognizing the symlink from "mplayer" to "mplayer2" (the package has an option to do that so that users don't have to reconfigure anything; they then use mplayer2 as a drop-in replacement for mplayer.)
In the SMPlayer preference, I had "mplayer" in the "MPlayer executable" setting. As soon as I changed that to "mplayer2", it started working. I changed it back to "mplayer" now and it keeps working. Looks like some weird bug with the workaround being changing that string to "mplayer2", playing a video, and then changing it back to "mplayer".