diff --git a/3rdparty/everest/library/Hacl_Curve25519_joined.c b/3rdparty/everest/library/Hacl_Curve25519_joined.c
index ee62be1ce..957294f64 100644
--- a/3rdparty/everest/library/Hacl_Curve25519_joined.c
+++ b/3rdparty/everest/library/Hacl_Curve25519_joined.c
@@ -18,6 +18,15 @@
  *
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
+#ifndef _BSD_SOURCE
+/* Required to get htole64() from gcc/glibc's endian.h (older systems)
+ * when we compile with -std=c99 */
+#define _BSD_SOURCE
+#endif
+#ifndef _DEFAULT_SOURCE
+/* (modern version of _BSD_SOURCE) */
+#define _DEFAULT_SOURCE
+#endif
 
 #include "common.h"
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7550f4593..cac13bd4e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -170,6 +170,9 @@ string(REGEX MATCH "Clang" CMAKE_COMPILER_IS_CLANG "${CMAKE_C_COMPILER_ID}")
 
 include(CheckCCompilerFlag)
 
+set(CMAKE_C_EXTENSIONS OFF)
+set(CMAKE_C_STANDARD 99)
+
 if(CMAKE_COMPILER_IS_GNU)
     # some warnings we want are not available with old GCC versions
     # note: starting with CMake 2.8 we could use CMAKE_C_COMPILER_VERSION
diff --git a/programs/test/udp_proxy.c b/programs/test/udp_proxy.c
index fd71e18fa..e3386d189 100644
--- a/programs/test/udp_proxy.c
+++ b/programs/test/udp_proxy.c
@@ -73,9 +73,10 @@ int main( void )
 #endif
 #endif /* _MSC_VER */
 #else /* ( _WIN32 || _WIN32_WCE ) && !EFIX64 && !EFI32 */
-#if defined(MBEDTLS_HAVE_TIME)
+#if defined(MBEDTLS_HAVE_TIME) || (defined(MBEDTLS_TIMING_C) && !defined(MBEDTLS_TIMING_ALT))
 #include <sys/time.h>
 #endif
+#include <sys/select.h>
 #include <sys/types.h>
 #include <unistd.h>
 #endif /* ( _WIN32 || _WIN32_WCE ) && !EFIX64 && !EFI32 */