I got trouble building eLua:

$ cd /tmp/elua/ ; ./build_elua.lua board=sim
[CONFIG] Found board description file at boards/known/sim.lua
[CONFIG] Generated board header file at boards/headers/board_sim.h
WARNING: unable to determine version from repository

---

Compiling eLua ...
CPU: LINUX
Board: sim
Platform: sim
Allocator: newlib
Boot Mode: standard
Target: lua
Toolchain: i686-gcc
ROMFS mode: verbatim
Version: unknown

---

[builder] Forcing rebuild due to configuration change.
[PHONY] #phony:romfs
Building ROM file system ...
Done, total size is 1 bytes
[COMPILE] .build/sim/src__buf.o
In file included from inc/platform.h:6:0,
from inc/platform_conf.h:9,
from src/buf.c:3:
inc/newlib/devman.h:7:19: fatal error: reent.h: No such file or directory
#include <reent.h>
^
compilation terminated.
[builder] Error building target
[builder] Last executed command was:
i586-alt-linux-gcc -DELUA_CPU_HEADER=""cpu_linux.h"" -DELUA_BOARD_HEADER=""board_sim.h"" -DELUA_CPU=LINUX -DELUA_BOARD=SIM -DELUA_PLATFORM=SIM -D__BUFSIZ__=128 -DELUA_CPU_LINUX -DELUA_BOARD_SIM -DELUA_PLATFORM_SIM -DLUA_PACK_VALUE -DELUA_ENDIAN_LITTLE -DELUA_SIMULATOR -DELUA_SIM_LINUX -DLUA_OPTIMIZE_MEMORY=2 -Iboards/headers -Iinc -Iinc/newlib -Iinc/remotefs -Isrc/platform -Isrc/lua -Isrc/modules -Isrc/platform/sim -Isrc/platform/sim/cpus -Isrc/uip -Isrc/fatfs -O0 -g -ffunction-sections -fdata-sections -fno-strict-aliasing -Wall -march=i386 -mfpmath=387 -m32 -fno-builtin -fno-stack-protector -c -MD -MF .build/sim/src__buf.d -o .build/sim/src__buf.o src/buf.c

I suggest that the header from newlib is missing.