RFC 7541 requires that dynamic table size update must occur at the
beginning of the first header block, and is signaled as SETTINGS
acknowledgement.  This commit checks these conditions.  If dynamic
table size update appears other than the beginning of the first header
block, it is treated as error.  If SETTINGS ACK is received, and next
HEADERS header block does not have dynamic table size update, it is
treated as error.

When you don't generated documentation via make html

The workaround introduced in a23ab006
may cause a problem in certain platform (e.g., MacPorts, see GH-303)