Standardize `-std=c++* as `CXXSTD`

Standardize `-std=c++* as `CXXSTD`

CXXSTD was added as the C++ analogue to CSTD.

CXXSTD defaults to `-std=c++11` with supporting compilers; `-std=gnu++98`,
otherwise for older versions of g++.

This change standardizes the CXXSTD variable, originally added to
googletest.test.inc.mk as part of r345203.

As part of this effort, convert all `CXXFLAGS+= -std=*` calls to use `CXXSTD`.

Notes:

This value is not sanity checked in bsd.sys.mk, however, given the two
most used C++ compilers on FreeBSD (clang++ and g++) support both modes, it is
likely to work with both toolchains. This method will be refined in the future
to support more variants of C++, as not all versions of clang++ and g++ (for
instance) support C++14, C++17, etc.

Any manual appending of `-std=*` to `CXXFLAGS` should be replaced with CXXSTD.
Example:

Before this commit:
```
CXXFLAGS+=	-std=c++14
```

After this commit:
```
CXXSTD=	c++14
```

Reviewed by:	asomers
Approved by:	emaste (mentor)
MFC after:	1 month
MFC with:	r345203, r345704, r345705
Relnotes:	yes
Tested with:	make tinderbox
Differential Revision:	https://reviews.freebsd.org/D19732