Fixes&updates for tlmu-1.4.0 after trying it

Hi,

I'm interest in your project hdlab/tlmu. After trying build and run the code on my test bench, I've made little update commits to it. As the followings:

1. fix vnc-tls compilation with newer versions of GNU-TLS
  I found the code can't compile correctly in case that the current version of GNU-TLS changes an interface
2. tlmu:fixed makefile for tlmu-doc.html generation
  after make && make tlmu, I found the tlmu-doc.html is not generated, so this commit fix the issue
3.  tlmu:modern SystemC includes TLM, so removed duplicate includings
  now that SystemC releases together with TLM-2.0, there is dumplicate including of it
4.  tlmu:fix SDL multithreading bug by temporary add '-nographic'
  I found there's an issue with multi-threading SDL call, so here using arg '-nographic' to avoid it when launching with qemu. Maybe this is an temporary solution, any better one may carry on in the future.

After all, I will keep on watching on this nice project TLMu and try my best to make it better, as well as, up-to-date with QEMU mainstream.

Thank you for your work!

LI Yi