src: Use gnu++17 as the default C++ standard

Previously the compiler's default C++ standard was used unlike C where
bsd.sys.mk explicitly sets a default language version.  Setting an
explicit default version will give a more uniform experience across
different compilers and compiler versions.

gnu++17 was chosen to match the default C standard.  It is well
supported by a wide range of clang (5+) and GCC (9+) versions.

gnu++17 is also the default C++ standard in recent versions of clang
(16+) and GCC (11+).  As a result, many of the explicit CXXSTD
settings in Makefiles had the effect of lowering the C++ standard
instead of raising it as was originally intended and are removed.

Note that the remaining explicit CXXSTD settings for atf and liblutok
explicitly lower the standard to C++11 due to use of the deprecated
auto_ptr<> template which is removed in later versions.

Reviewed by:	imp, asomers, dim, emaste
Differential Revision:	https://reviews.freebsd.org/D49223

lib/clang/llvm.build.mk

@@ -119,7 +119,6 @@ LDFLAGS+=	-Wl,-m,elf64lriscv_fbsd
 .endif
 .endif
 
-CXXSTD?=	c++17
 CXXFLAGS+=	-fno-exceptions
 CXXFLAGS+=	-fno-rtti
 .if ${.MAKE.OS} == "FreeBSD" || !defined(BOOTSTRAPPING)

lib/libc/tests/stdlib/Makefile

@@ -59,8 +59,6 @@ PROGS+=		h_getopt h_getopt_long
 
 CFLAGS+=	-I${.CURDIR}
 
-CXXSTD.cxa_thread_atexit_test=	c++11
-CXXSTD.cxa_thread_atexit_nothr_test=	c++11
 LIBADD.cxa_thread_atexit_test+=		pthread
 
 # Tests that requires Blocks feature

lib/libclang_rt/Makefile.inc

@@ -33,4 +33,3 @@ CXXFLAGS+=	-fvisibility-inlines-hidden
 CXXFLAGS+=	-fvisibility=hidden
 CFLAGS+=	-I${CRTSRC}/include
 CFLAGS+=	-I${CRTSRC}/lib
-CXXSTD=		c++17

lib/libcxxrt/Makefile

@@ -23,7 +23,6 @@ SRCS+=		typeinfo.cc
 
 WARNS?=		0
 CFLAGS+=	-isystem ${SRCDIR} -nostdinc++
-CXXSTD?=	c++14
 
 .if exists(Version.map.${MACHINE})
 VERSION_MAP=	${.CURDIR}/Version.map.${MACHINE}

lib/libgcc_eh/Makefile.inc

@@ -38,7 +38,6 @@ CFLAGS+=	-I${UNWINDINCDIR}
 CFLAGS+=	-D_LIBUNWIND_IS_NATIVE_ONLY
 CFLAGS+=	-D_LIBUNWIND_USE_FRAME_HEADER_CACHE
 CXXFLAGS+=	-fno-rtti
-CXXSTD?=	c++11
 STATIC_CXXFLAGS+= -fvisibility=hidden -fPIC
 # Probably need to just move this earlier or use CXXFLAGS
 .if ${MK_DIRDEPS_BUILD} == "yes"

lib/libomp/Makefile

@@ -57,7 +57,6 @@ CFLAGS+=	-fdata-sections
 CXXFLAGS+=	-fvisibility-inlines-hidden
 CXXFLAGS+=	-fno-exceptions
 CXXFLAGS+=	-fno-rtti
-CXXSTD=		c++17
 
 .if ${COMPILER_TYPE} == "clang"
 .if ${MACHINE_CPUARCH} == "i386"

lib/ofed/libibnetdisc/Makefile

@@ -29,7 +29,6 @@ LIBADD=		osmcomp ibmad ibumad
 CFLAGS+=	-DHAVE_CONFIG_H=1
 CFLAGS+=	-I${_spath}
 CFLAGS+=	-I${SYSROOT:U${DESTDIR}}/${INCLUDEDIR}/infiniband
-CXXSTD=		c++11
 VERSION_MAP=	${_spath}/libibnetdisc.map
 
 .include <bsd.lib.mk>

libexec/atf/atf-pytest-wrapper/Makefile

@@ -4,7 +4,6 @@
 PACKAGE=	tests
 PROG_CXX=	atf_pytest_wrapper
 SRCS=		atf_pytest_wrapper.cpp
-CXXSTD=		c++17
 MAN=
 
 .include <bsd.prog.mk>

share/mk/bsd.sys.mk

@@ -22,6 +22,8 @@ CFLAGS+=	-std=iso9899:1999
 CFLAGS+=	-std=${CSTD}
 .endif # CSTD
 
+CXXSTD?=	gnu++17
+
 .if !empty(CXXSTD)
 CXXFLAGS+=	-std=${CXXSTD}
 .endif

share/mk/googletest.test.inc.mk

@@ -9,6 +9,4 @@ GTESTS_CXXFLAGS+= -frtti
 # libgmock's, etc, headers.
 CXXFLAGS+=	-I${DESTDIR}${INCLUDEDIR}/private
 
-CXXSTD?=	c++14
-
 NO_WTHREAD_SAFETY=

tests/sys/fs/fusefs/Makefile

@@ -89,7 +89,6 @@ CXXFLAGS+=	-I${SRCTOP}/tests
 CXXFLAGS+=	-I${FUSEFS}
 CXXFLAGS+=	-I${MOUNT}
 .PATH:		${MOUNT}
-CXXSTD=		c++14
 
 LIBADD+=	pthread
 LIBADD+=	gmock gtest

usr.bin/dtc/Makefile

@@ -6,8 +6,6 @@ WARNS?=	3
 
 CXXFLAGS+=	-fno-rtti -fno-exceptions
 
-CXXSTD=	c++17
-
 NO_SHARED?=NO
 
 .include <bsd.prog.mk>

usr.sbin/pmc/Makefile

@@ -2,7 +2,6 @@
 
 PROG_CXX=	pmc
 MAN=	
-CXXSTD= c++14
 CWARNFLAGS.gcc+= -Wno-redundant-decls
 CFLAGS+= -I${SRCTOP}/lib/libpmcstat