Plymouth isn't really designed to be built from source and get installed by end users. For it to work correctly, it needs integration with the distribution. Because it starts so early, it needs to be packed into the distribution's initial ram disk, and the distribution needs to poke plymouth to tell it how boot is progressing.
Unfortunately, there isn't great documentation for distributors. In the mean time, this email exchange might be useful:
Hi,
> I recently found a video showing Fedora booting with plymouth and I
> liked it. So I wanted to use plymouth with my favorite distri (gentoo).
> I know I'm enduser and the info text on freedesktop.org says plymouth
> is not meant to be installed by end users but I want to give it a try.
> I have KMS running and am using my own initramfs anyway (encrypted root
> disk) so how hard can it be? Can anybody please give me some straight
> forward hints on how to use plymouth? (The INSTALL file is empty and
> the README file is boring)
So the basic idea is you need to:
1) build plymouth into the system. On fedora we use these conffigure flags: