add a simple Makefile to build and clean up

add missing macros, static inline functions, etc.
add README to illustrate the purpose of this repo.

Signed-off-by: Yonghong Song <yhs@fb.com>

scripts/check-reallocarray.sh

@@ -0,0 +1,18 @@
+#/bin/sh
+
+tfile=$(mktemp /tmp/test_reallocarray_XXXXXXXX.c)
+ofile=${tfile%.c}.o
+
+cat > $tfile <<EOL
+#define _GNU_SOURCE
+#include <stdlib.h>
+
+int main(void)
+{
+	return !!reallocarray(NULL, 1, 1);
+}
+EOL
+
+gcc $tfile -o $ofile >& /dev/null
+if [ $? -ne 0 ]; then echo "FAIL"; fi
+/bin/rm -f $tfile $ofile