Make THUNARX_DIRS compile-time

Hi, looking at the recently released 4.17.8 I noticed the fix for #748 (closed) which adds a way to make thunar load plugins from user-supplied directories through an environment variable.

I can understand the need to have plugins in other directories (in Debian we actually carry a patch so plugins can be loaded from the multiarch dir /usr/lib/ or from non multiarch /usr/lib/thunarx-3: https://sources.debian.org/src/thunar/4.17.7-1/debian/patches/01_support-non-multiarch-modules.patch/) but I'm a bit annoyed by the fact that it's user-provided from an env var.

For security reason I think where Thunar can load code from should be fixed at compile time (which I think would still work for #748 (closed)) and not runtime, so this can't be abused. I don't have examples handy but I think it's better to be safe than sorry.