Remove remaining uses of ${COMPILER_FEATURES:Mc++11}

[arichardson]

Summary

All supported compilers have C++11 support so these checks can be replaced
with MK_CXX guards.

See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252759

[emaste]

usr.bin/Makefile

215

We should extend the WITHOUT_CXX description in src.conf(5) - I'm not sure we need to keep it up-to-date on an ongoing basis, but it currently suggests that gperf(1) and devd(8) are the only things it will turn off.