[mesa-libgl]'s libGL.so links against libglapi.so, which is part of the [mesa] package, but [mesa-libgl] declares no dependency on it. This means that in a clean chroot, when building something against libgl, it will fail (unless some other dependency happens to depend on [mesa], or the package builder is fooled into believing that the package itself should depend on [mesa]).

One should look at the provides/depends/conflicts lists from the Arch package before they started applying the libglvnd patches.