Update doc for Debian 7.0 or above and add g++ requirement

The encoder is not required to send dynamic table size update if the
table size is not changed from the previous value after accepting new
maximum value.

This will improve performance since we can avoid indirect call of
internal functions.  The downside is we now require libnghttp2 static
library to run unit tests.

Merge branch 'nghttpx-tls-dyn-rec-config' of https://github.com/LPardue/nghttp2 into LPardue-nghttpx-tls-dyn-rec-config