deque: check HAVE_STATEMENT_EXPR, tweaks from feedback

Thanks to the detailed feedback from David Gibson, I made the
following improvements:

* add missing includes
* check for statement expression support, give an error if absent
* ccanlint directive to skip "without features" steps
* add license ref to top of source files
* rename run1.c test to api1.c

Signed-off-by: Dan Good <dan@dancancode.com>