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

1 files changed, 0 insertions, 4 deletions

diff --git a/share/mk/googletest.test.inc.mk b/share/mk/googletest.test.inc.mk
index 84959658bdbd..98f0e6a6fbe3 100644
--- a/share/mk/googletest.test.inc.mk
+++ b/share/mk/googletest.test.inc.mk
@@ -1,13 +1,9 @@
 # $FreeBSD$
 
-# XXX: this should be defined in bsd.sys.mk
-CXXSTD?=	c++11
-
 GTESTS_CXXFLAGS+= -DGTEST_HAS_POSIX_RE=1
 GTESTS_CXXFLAGS+= -DGTEST_HAS_PTHREAD=1
 GTESTS_CXXFLAGS+= -DGTEST_HAS_STREAM_REDIRECTION=1
 GTESTS_CXXFLAGS+= -frtti
-GTESTS_CXXFLAGS+= -std=${CXXSTD}
 
 # XXX: src.libnames.mk should handle adding this directory for libgtest's,
 # libgmock's, etc, headers.