Bump version to 1.6
Add the shtk_config_include function

Allow configuration files to include other files while supporting
relative paths.  This is, currently, for the benefit of FreeBSD's
autotest scripts where I'm finding that the majority of the
per-machine configuration files are just copy/pastes of each other.
Respect silent mode in custom rules

Follow the instructions in the automake documentation to properly silence
commands in custom rules when silent mode is enabled.