Fix up CXXSTD support originally added in r345708

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. This avoids the need for having
to bake NOP default into bsd.sys.mk for supported compiler-toolchain
versions.

In order to make this change possible, add CXXSTD to Makefile snippets which
relied on the default value (c++11) added in r345708.

MFC after:      2 weeks
MFC with:       r345708, r346574
Reviewed by:    emaste
Reported by:    jbeich
Differential Revision: https://reviews.freebsd.org/D19895 (as part of a larger change)

share/mk/bsd.sys.mk

@@ -25,17 +25,9 @@ CFLAGS+=	-std=iso9899:1999
 CFLAGS+=	-std=${CSTD}
 .endif # CSTD
 
-.if ${COMPILER_FEATURES:Mc++11}
-CXXSTD?=	c++11
-.elif ${COMPILER_TYPE} == "gcc"
-# Prior versions of g++ support C++98 with GNU extensions by default.
-CXXSTD?=	gnu++98
-.else
-# Assume that the compiler supports at least C++98.
-CXXSTD?=	c++98
-.endif
+.if !empty(CXXSTD)
 CXXFLAGS+=	-std=${CXXSTD}
-# CXXSTD
+.endif
 
 # -pedantic is problematic because it also imposes namespace restrictions
 #CFLAGS+=	-pedantic

share/mk/googletest.test.inc.mk

@@ -9,4 +9,6 @@ GTESTS_CXXFLAGS+= -frtti
 # libgmock's, etc, headers.
 CXXFLAGS+=	-I${DESTDIR}${INCLUDEDIR}/private
 
+CXXSTD?=	c++11
+
 NO_WTHREAD_SAFETY=