luarocks: fix lua modules building with luajit

currently, when luajit is selected as luainterpreter,
all lua module fails to build with the message:
    Error: Failed finding Lua library. You may need to configure LUA_LIBDIR.

see http://autobuild.buildroot.net/results/66de049c306d7aff66894d53e2f4fa755fd01594



Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>