TLS examples requires `--enable-writedup`

[dgarske]

The make fails if the write-dup feature is not enabled. Can we change this so it builds with a main() stub and printf an error at run-time instead of failing build-time with #error. This will allow other TLS examples to build and run.

gcc -o client-tls-writedup client-tls-writedup.c -Wall -I/usr/local/include -Os -pthread -L/usr/local/lib -lm -lwolfssl
client-tls-writedup.c:43:6: error: #error "wolfSSL must be configured and installed with --enable-writedup"
     #error "wolfSSL must be configured and installed with --enable-writedup"
      ^~~~~
make: *** [client-tls-writedup] Error 1

[levirak]

I've added a fix for this to pull request #63, which among other things adds the writedup section to the README's tutorial section. If you'd rather I make a new pull request for just this, let me know.

[dgarske]

I reviewed and tested your fix and it looks good. A-okay to include in existing PR. For this example using the #warning is okay, however its worth noting that the Visual Studio compiler doesn't support it and uses the #pragma warning.