Add missing macro existence check in byte swapping code in bignum.c
@@ -741,7 +741,7 @@ static mbedtls_mpi_uint mpi_uint_bigendian_to_host( mbedtls_mpi_uint x )
#if ( __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ )
/* For GCC and Clang, have builtins for byte swapping. */
-#if( defined(__GNUC__) && __GNUC_PREREQ(4,3) )
+#if( defined(__GNUC__) && defined(__GNUC_PREREQ) && __GNUC_PREREQ(4,3) )
#define have_bswap
#elif defined(__clang__) && \
defined(__has_builtin) && \
