diff --git a/src/common/sha3/xkcp_dispatch.h b/src/common/sha3/xkcp_dispatch.h
index 15103dbe61..73bedab69f 100644
--- a/src/common/sha3/xkcp_dispatch.h
+++ b/src/common/sha3/xkcp_dispatch.h
@@ -13,6 +13,12 @@
 #ifndef OQS_SHA3_XKCP_DISPATCH_H
 #define OQS_SHA3_XKCP_DISPATCH_H
 
+#if defined __has_include
+#if __has_include (<pthread.h>)
+#define OQS_SHA3_XKCP_DISPATCH_USE_PTHREAD
+#endif
+#endif
+
 typedef void KeccakInitFn(void *);
 extern KeccakInitFn \
 KeccakP1600_Initialize, \
diff --git a/src/common/sha3/xkcp_sha3.c b/src/common/sha3/xkcp_sha3.c
index 16ee2ff5c0..f2daca3796 100644
--- a/src/common/sha3/xkcp_sha3.c
+++ b/src/common/sha3/xkcp_sha3.c
@@ -13,21 +13,21 @@
 
 #include <oqs/common.h>
 
+#ifdef OQS_SHA3_XKCP_DISPATCH_USE_PTHREAD
+#include <pthread.h>
+#endif
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
-#ifndef __STDC_NO_THREADS__
-#include <threads.h>
-#endif
 
 #define KECCAK_CTX_ALIGNMENT 32
 #define _KECCAK_CTX_BYTES (200+sizeof(uint64_t))
 #define KECCAK_CTX_BYTES (KECCAK_CTX_ALIGNMENT * \
   ((_KECCAK_CTX_BYTES + KECCAK_CTX_ALIGNMENT - 1)/KECCAK_CTX_ALIGNMENT))
 
-#ifndef __STDC_NO_THREADS__
-static once_flag dispatch_once_flag = ONCE_FLAG_INIT;
+#ifdef OQS_SHA3_XKCP_DISPATCH_USE_PTHREAD
+static pthread_once_t dispatch_once_control = PTHREAD_ONCE_INIT;
 #endif
 
 static KeccakInitFn *Keccak_Initialize_ptr = NULL;
@@ -85,8 +85,8 @@ static void Keccak_Dispatch() {
  *                that have not been permuted, or not-yet-squeezed bytes.
  **************************************************/
 static void keccak_inc_reset(uint64_t *s) {
-#ifndef __STDC_NO_THREADS__
-	call_once(&dispatch_once_flag, Keccak_Dispatch);
+#ifdef OQS_SHA3_XKCP_DISPATCH_USE_PTHREAD
+	pthread_once(&dispatch_once_control, Keccak_Dispatch);
 #else
 	if (Keccak_Initialize_ptr == NULL) {
 		Keccak_Dispatch();
diff --git a/src/common/sha3/xkcp_sha3x4.c b/src/common/sha3/xkcp_sha3x4.c
index c6eca5d8b2..a5bd03bc5c 100644
--- a/src/common/sha3/xkcp_sha3x4.c
+++ b/src/common/sha3/xkcp_sha3x4.c
@@ -8,21 +8,21 @@
 #include <oqs/common.h>
 #include <oqs/oqsconfig.h>
 
+#ifdef OQS_SHA3_XKCP_DISPATCH_USE_PTHREAD
+#include <pthread.h>
+#endif
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
-#ifndef __STDC_NO_THREADS__
-#include <threads.h>
-#endif
 
 #define KECCAK_X4_CTX_ALIGNMENT 32
 #define _KECCAK_X4_CTX_BYTES (800+sizeof(uint64_t))
 #define KECCAK_X4_CTX_BYTES (KECCAK_X4_CTX_ALIGNMENT * \
   ((_KECCAK_X4_CTX_BYTES + KECCAK_X4_CTX_ALIGNMENT - 1)/KECCAK_X4_CTX_ALIGNMENT))
 
-#ifndef __STDC_NO_THREADS__
-static once_flag dispatch_once_flag = ONCE_FLAG_INIT;
+#ifdef OQS_SHA3_XKCP_DISPATCH_USE_PTHREAD
+static pthread_once_t dispatch_once_control = PTHREAD_ONCE_INIT;
 #endif
 
 static KeccakX4InitFn *Keccak_X4_Initialize_ptr = NULL;
@@ -65,8 +65,8 @@ static void Keccak_X4_Dispatch() {
 }
 
 static void keccak_x4_inc_reset(uint64_t *s) {
-#ifndef __STDC_NO_THREADS__
-	call_once(&dispatch_once_flag, Keccak_X4_Dispatch);
+#ifdef OQS_SHA3_XKCP_DISPATCH_USE_PTHREAD
+	pthread_once(&dispatch_once_control, Keccak_X4_Dispatch);
 #else
 	if (Keccak_X4_Initialize_ptr == NULL) {
 		Keccak_X4_Dispatch();