diff --git a/configure.ac b/configure.ac
index e05f2023..0686f9f5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -446,21 +446,26 @@ AC_SUBST(HAVE_AVX_ASM_V)
 AC_MSG_CHECKING(for 128-bit arithmetic)
 HAVE_TI_MODE_V=0
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
-#ifndef __GNUC__
-# error mode(TI) is a gcc extension
+#if !defined(__GNUC__) && !defined(__SIZEOF_INT128__)
+# error mode(TI) is a gcc extension, and __int128 is not available
 #endif
 #if defined(__clang__) && !defined(__x86_64__)
-# error clang does not properly compile smult_curve25519_donna_c64.c
+# error clang does not properly handle the 128-bit type on 32-bit systems
 #endif
 #ifndef NATIVE_LITTLE_ENDIAN
-# error donna_c64 currently requires a little endian CPU
+# error libsodium currently expects a little endian CPU for the 128-bit type
 #endif
 #ifdef __EMSCRIPTEN__
 # error emscripten currently supports only shift operations on integers \
 #       larger than 64 bits
 #endif
+#include <stddef.h>
 #include <stdint.h>
+#if defined(__SIZEOF_INT128__)
+typedef unsigned __int128 uint128_t;
+#else
 typedef unsigned uint128_t __attribute__((mode(TI)));
+#endif
 void fcontract(uint128_t *t) {
   *t += 0x8000000000000 - 1;
 }