Try to detect runtime when specifying INC, LIB and BIN flags.

When passing /INC /LIB and /BIN flags, the runtime used by the supplied
Lua installation is not probed, so vars.LUA_RUNTIME remains being nil,
which causes an error.
Merge pull request #136 from ignacio/install_runtime_fix

Try to detect runtime when specifying INC, LIB and BIN flags.