r345708 worked for the base system, but unfortunately, caused a lot of
disruption for third-party packages that relied on C++, since bsd.sys.mk is
used by applications outside the base system. The defaults picked didn't match
the compiler's defaults and broke some builds that didn't specify a standard,
as well as some that overrode the value by setting `-std=gnu++14` (for
example) manually.
This change takes a more relaxed approach to appending `-std=${CXXSTD}` to
CXXFLAGS, by only doing so when the value is specified, as opposed to
overriding the standard set by an end-user.
In order to make this change possible, add CXXSTD in Makefile snippets which
relied on the default value (c++11) added in r345708.
MFC after: 2 weeks
MFC with: r345708
Reported by: jbeich