Remove remaining uses of ${COMPILER_FEATURES:Mc++11}

All supported compilers have C++11 support so these checks can be replaced
with MK_CXX guards.
See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252759

PR:		252759
Reviewed By:	emaste
Differential Revision: https://reviews.freebsd.org/D28234

lib/libc/tests/stdlib/Makefile

@@ -11,10 +11,8 @@ ATF_TESTS_C+=		qsort_s_test
 ATF_TESTS_C+=		set_constraint_handler_s_test
 ATF_TESTS_C+=		strfmon_test
 ATF_TESTS_C+=		tsearch_test
-.if ${COMPILER_FEATURES:Mc++11}
 ATF_TESTS_CXX+=		cxa_thread_atexit_test
 ATF_TESTS_CXX+=		cxa_thread_atexit_nothr_test
-.endif
 
 # All architectures on FreeBSD have fenv.h
 CFLAGS+=	-D__HAVE_FENV

lib/ofed/libibnetdisc/Makefile

@@ -31,9 +31,7 @@ LIBADD=		osmcomp ibmad ibumad
 CFLAGS+=	-DHAVE_CONFIG_H=1
 CFLAGS+=	-I${_spath}
 CFLAGS+=	-I${SYSROOT:U${DESTDIR}}/${INCLUDEDIR}/infiniband
-.if ${COMPILER_FEATURES:Mc++11}
 CXXSTD=		c++11
-.endif
 VERSION_MAP=	${_spath}/libibnetdisc.map
 
 .include <bsd.lib.mk>

usr.bin/Makefile

@@ -212,9 +212,7 @@ SUBDIR.${MK_GAMES}+=	number
 SUBDIR.${MK_GAMES}+=	pom
 SUBDIR.${MK_GAMES}+=	primes
 SUBDIR.${MK_GAMES}+=	random
-.if ${COMPILER_FEATURES:Mc++11}
-SUBDIR+=	dtc
-.endif
+SUBDIR.${MK_CXX}+=	dtc
 .if ${MK_GH_BC} == "yes"
 SUBDIR+=		gh-bc
 .else

usr.sbin/Makefile

@@ -186,9 +186,7 @@ SUBDIR.${MK_NTP}+=	ntp
 SUBDIR.${MK_OPENSSL}+=	keyserv
 SUBDIR.${MK_PF}+=	ftp-proxy
 SUBDIR.${MK_PKGBOOTSTRAP}+=	pkg
-.if ${COMPILER_FEATURES:Mc++11}
-SUBDIR.${MK_PMC}+=	pmc
-.endif
+SUBDIR.${MK_PMC}.${MK_CXX}+=	pmc
 SUBDIR.${MK_PMC}+=	pmcannotate pmccontrol pmcstat pmcstudy
 SUBDIR.${MK_PORTSNAP}+=	portsnap
 SUBDIR.${MK_PPP}+=	ppp