net/yami4: fix build

Add USES=compiler:c++11-lang to use new GCC and add COMPILER="${CXX}" to actually respect that.

Errors:
value_publisher.h: At global scope:
value_publisher.h:180: error: ISO C++ forbids declaration of 'unique_ptr' with no type
value_publisher.h:180: error: invalid use of '::'
value_publisher.h:180: error: expected ';' before '<' token
value_publisher.h:182: error: ISO C++ forbids declaration of 'unique_ptr' with no type
value_publisher.h:182: error: invalid use of '::'
value_publisher.h:182: error: expected ';' before '<' token

Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20600