Oh, the joy of compilers pretending to support C99, but that actually don't

configure.ac

@@ -189,6 +189,7 @@ dnl Checks
 AC_PROG_CC_C99
 AM_PROG_AS
 AC_USE_SYSTEM_EXTENSIONS
+AC_C_VARARRAYS
 
 AC_CHECK_DEFINE([__native_client__], [NATIVECLIENT="yes"], [])
 

src/libsodium/sodium/utils.c

@@ -128,7 +128,7 @@ sodium_stackzero(const size_t len)
 {
 #ifdef HAVE_ALLOCA
     sodium_memzero(alloca(len), len);
-#elif __STDC_VERSION__ >= 199901L
+#elif HAVE_C_VARARRAYS
     unsigned char fodder[len];
     sodium_memzero(fodder, len);
 #endif