Support libexec and lib64 paths for the webkitgtk helper

[Conan-Kudo]

The helper should be installed in the libexec directory path, so we should default to that.

Additionally, support more permutations of libexec paths so that this is more likely to work out of the gate.

[fzwoch]

Huh, is this still alive? :D

These look like sensible changes, thanks!

[Conan-Kudo]

I'm shipping it as obs-studio-plugin-webkitgtk in Fedora, since it works on all of Fedora's architectures and is a great basic browser plugin.