diff --git a/Makefile b/Makefile
index 8eadf7a..e542a5f 100644
--- a/Makefile
+++ b/Makefile
@@ -41,7 +41,7 @@ CC ?= gcc
 PKG_CONFIG ?= pkg-config
 CFLAGS ?= -ansi -O0 -g3 -Wall -Wextra -Werror -Wconversion \
 	-Wstrict-prototypes -Wno-unused-parameter -pedantic
-CFLAGS += -fPIC -DMPACK_DEBUG_REGISTRY_LEAK
+CFLAGS += -fPIC -std=c99 -DMPACK_DEBUG_REGISTRY_LEAK
 ifeq ($(MPACK_LUA_VERSION_NOPATCH),5.3)
 # Lua 5.3 has integer type, which is not 64 bits for -ansi since c89 doesn't
 # have `long long` type.
diff --git a/README.md b/README.md
index 45339b7..53aabfe 100644
--- a/README.md
+++ b/README.md
@@ -2,3 +2,15 @@
 
 [![Travis Build Status](https://travis-ci.org/libmpack/libmpack-lua.svg?branch=master)](https://travis-ci.org/libmpack/libmpack-lua)
 
+## Building
+
+```bash
+LUA_TARGET=$(PLATFORM) make
+```
+
+Where `PLATFORM` is a supported Lua 5.1 platform: linux (default), freebsd, macosx, ...
+
+For the complete list of targets run:
+```bash
+make; cd .deps/5.1.5/src/lua && make
+```
diff --git a/lmpack.c b/lmpack.c
index 2fbd855..fa22989 100644
--- a/lmpack.c
+++ b/lmpack.c
@@ -16,7 +16,12 @@
  */
 #define LUA_LIB
 /* for snprintf */
+#ifdef __APPLE__
+// Apple uses C99 source, not XOPEN
+#define _C99_SOURCE 1
+#else
 #define _XOPEN_SOURCE 500
+#endif
 #include <limits.h>
 #include <stdlib.h>
 #include <string.h>