diff --git a/Makefile b/Makefile
index 96aa71b..b06a946 100644
--- a/Makefile
+++ b/Makefile
@@ -5,7 +5,7 @@ all: gps-sdr-sim
 
 SHELL=/bin/bash
 CC=gcc
-CFLAGS=-fopenmp -O3
+CFLAGS=-std=c99 -fopenmp -O3
 LDFLAGS=-lm -fopenmp
 
 gps-sdr-sim: gpssim.o
diff --git a/gpssim.c b/gpssim.c
index 210e43e..21a3992 100644
--- a/gpssim.c
+++ b/gpssim.c
@@ -9,7 +9,7 @@
 #ifdef _WIN32
 #include "getopt.h"
 #else
-#include <unistd.h>
+#include <getopt.h>
 #endif
 
 #ifndef bool