www/swish++: Fix build with Clang 6

option_stream.c:272:14: error: assigning to 'char *' from incompatible type 'char'

o.arg_ = '\0';
         ^~~~

option_stream.c:286:30: warning: '/*' within block comment [-Wcomment]
/*#define TEST_OPTION_STREAM /**/

^

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475044_s336572/logs/swish++-6.1.5_3.log

• While here deprecate it. Upstream development seems to be pretty dead.