Make use of static_assert conditional on whether it's available.

wasmtime-wasi/sandboxed-system-primitives/src/posix.c

@@ -46,6 +46,9 @@
 #include "rights.h"
 #include "str.h"
 
+// static_assert is not defined in older compilers.
+#ifdef static_assert
+
 // struct iovec must have the same layout as __wasi_iovec_t.
 static_assert(offsetof(struct iovec, iov_base) ==
                   offsetof(__wasi_iovec_t, buf),
@@ -78,6 +81,8 @@ static_assert(sizeof(((struct iovec *)0)->iov_len) ==
 static_assert(sizeof(struct iovec) == sizeof(__wasi_ciovec_t),
               "Size mismatch");
 
+#endif
+
 #if defined(WASMTIME_SSP_STATIC_CURFDS)
 static __thread struct fd_table *curfds;
 static __thread struct fd_prestats *prestats;