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
760b1a815b