BUILD: link with libdl if needed for Lua support

BUILD: link with libdl if needed for Lua support

On platforms where the dl*() functions are not part of the libc, a
program linking Lua also needs to link to libdl.

Moreover, on platforms using a gold linker with the --as-needed flag,
the libdl library needs to be linked after linking Lua, otherwise, it
won't be marked as needed and will be discarded and its symbols won't be
present at the end of the linking phase.

Ubuntu enables the --as-needed flag by default. Other distributions may
advertise its use, like Gentoo.