Fix various CI build warnings

  * [-Wpointer-arith] arithmetic on a pointer to void is a GNU extension
  * [-Wswitch-enum] enumeration values 'E1, ...' not explicitly handled in switch
  * [-Wunused-parameter] unused parameter 'p'

For '-Wswitch-enum', the switch statements in the individual backends'
set_option() function has been removed. It is not expected that backends
will need to handle or be aware of all the options.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>