Libtermkey can be linked against unibilium or curses. For the bundled
dependencies Neovim links against static versions of libtermkey and
unibilium, after building both libraries.
However libtermkey requires pkg-config to be installed in order to detect
and link against unibilium, otherwise it falls back to curses by default.
In systems where pkg-config is not installed building Neovim against the
bundled libtermkey caused a linking error (#2484).
So pkg-config needs to be installed for the bundled libtermkey to build
properly.