* configury: load  local std modules as expected by mkrockspecs.lua.

Rather than having to manually update the installed lua-stdlib
behind luarocks' back, let the mkrockspecs.lua script find the
modules it was written for in the source tree.
* Makefile.am (LUA_PATH): Default to compiled-in search path.
(LUA_ENV): Load modules from the source tree instead of potentially
outdated versions from the lua installation.
(rockspecs): Run lua with LUA_ENV set.