For certain build systems (e.g. nix, guix, spack) we need to tell
libwhich about non-standard search paths. Currently the only way of
doing so is by setting LD_LIBRARY_PATH, which is difficult to scope just
to libwhich execution, and as a result can influence the runtime
behavior of other executables.
This patch allows you to set LIBWHICH_LIBRARY_PATH=x:y:z, which
libwhich uses to init LD_LIBRARY_PATH before it reruns itself, so
dlopen(...) can locate libraries in these search paths.