Treat warnings as errors for IAR

Fix IAR compiler warnings

Two warnings have been fixed:
1. code 'if( len <= 0xFFFFFFFF )' gave warning 'pointless integer comparison'.
   This was fixed by wraping the condition in '#if SIZE_MAX > 0xFFFFFFFF'.
2. code 'diff |= A[i] ^ B[i];' gave warning 'the order of volatile accesses is undefined in'.
   This was fixed by read the volatile data in temporary variables before the computation.

Explain IAR warning on volatile access

Consistent use of CMAKE_C_COMPILER_ID
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 60b431a..2b5a616 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -630,7 +630,13 @@
     volatile unsigned char diff = 0;
 
     for( i = 0; i < n; i++ )
-        diff |= A[i] ^ B[i];
+    {
+        /* Read volatile data in order before computing diff.
+         * This avoids IAR compiler warning:
+         * 'the order of volatile accesses is undefined ..' */
+        unsigned char x = A[i], y = B[i];
+        diff |= x ^ y;
+    }
 
     return( diff );
 }