CURRENT: buildworld fails

Since approx. a week for now, I can not build FreeBSD 10.0-CURRENT/amd64
anymore! This happens to be on ALL(!) FreeBSD 10 boxes around here I
maintain.
Build is usually performed with CLANG, but also legacy gcc 4.2.1 build
do fail.

The error is always the same, as documented below.

I allow to build with "WITH_BSD_SORT" in /etc/src.conf.

CFLAGS and COPTFLAGS are set to -pipe -O3 -fno-strict-aliasing
-march=native when compiling with CLANG, otherwise the standard is used
as introduced by the vanilla sources.

What I tried so far:

a) build and install kernel -> works
b) build and install /usr/src/lib via "make clean cleandepend depend obj
all install" doesn't work anymore, it fails with
."/usr/src/lib/Makefile", line 179: Malformed conditional (${MK_NAND} !=
"no")
"/usr/src/lib/Makefile", line 181: if-less endif

c) make installincludes from /usr/src works. But it doesn't relief as I
hoped.

As the error below may suggest, there seems to be an issue with the
libstc++ lib.
Building ports also fails due to errors refering to libstdc++.so.

I feel helpless at the moment since the problem seems only to be sticky
with me around here. Do others around here also allow the build of new
C++ stuff with
WITH_LIBCPLUSPLUS=      YES
in /etc/src.conf?

Regards,

Oliver
[...]
clang++ -O2 -fno-strict-aliasing -pipe -pipe -O3 -fno-strict-aliasing
-march=native -Qunused-arguments -fstack-protector -Wsystem-headers
-Wno-empty-body -Wno-string-plus-int -Wno-tautological-compare
-Wno-unused-value -Wno-parentheses-equality -Wno-unused-function
-Wno-conversion -Wno-switch -Wno-switch-enum -Wno-parentheses
-I/usr/src/gnu/usr.bin/gperf/../../../contrib/gperf/lib
-I/usr/src/gnu/usr.bin/gperf  -o gperf bool-array.o hash-table.o input.o
keyword-list.o keyword.o main.o options.o output.o positions.o search.o
version.o getline.o hash.o
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_ifstream<wchar_t, std::char_traits<wchar_t>
>::is_open() const'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_string<wchar_t, std::char_traits<wchar_t>,
std::allocator<wchar_t> >::_Rep::_M_set_length_and_sharable(unsigned long)'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_string<wchar_t, std::char_traits<wchar_t>,
std::allocator<wchar_t> >::_M_check_length(unsigned long, unsigned long,
char const*) const'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_fstream<char, std::char_traits<char>
>::is_open() const'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_istream<wchar_t, std::char_traits<wchar_t>
>::ignore()'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_string<wchar_t, std::char_traits<wchar_t>,
std::allocator<wchar_t> >::_M_copy(wchar_t*, wchar_t const*, unsigned long)'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::string::_M_assign(char*, unsigned long, char)'
/usr/obj/usr/src/tmp/usr/bin/ld: Warning: size of symbol
`_ZNSt13basic_istreamIwSt11char_traitsIwEE6ignoreEv_at_GLIBCXX_3.4' changed
from 243 in /usr/obj/usr/src/tmp/usr/lib/libstdc++.so to 211 in
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_fstream<wchar_t, std::char_traits<wchar_t>
>::is_open() const'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_string<wchar_t, std::char_traits<wchar_t>,
std::allocator<wchar_t> >::_M_move(wchar_t*, wchar_t const*, unsigned long)'
/usr/obj/usr/src/tmp/usr/bin/ld: Warning: size of symbol
`_ZNSbIwSt11char_traitsIwESaIwEE4_Rep26_M_set_length_and_sharableEm_at_GLIBCXX_3.4'
changed from 19 in /usr/obj/usr/src/tmp/usr/lib/libstdc++.so to 24 in
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::string::_M_move(char*, char const*, unsigned long)'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::istream::ignore()'
/usr/obj/usr/src/tmp/usr/bin/ld: Warning: size of symbol
`_ZNKSs15_M_check_lengthEmmPKc_at_GLIBCXX_3.4' changed from 39 in
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so to 36 in
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_ofstream<char, std::char_traits<char>
>::is_open() const'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_string<wchar_t, std::char_traits<wchar_t>,
std::allocator<wchar_t> >::_M_assign(wchar_t*, unsigned long, wchar_t)'
/usr/obj/usr/src/tmp/usr/bin/ld: Warning: size of symbol
`_ZNKSbIwSt11char_traitsIwESaIwEE15_M_check_lengthEmmPKc_at_GLIBCXX_3.4'
changed from 39 in /usr/obj/usr/src/tmp/usr/lib/libstdc++.so to 36 in
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so
/usr/obj/usr/src/tmp/usr/bin/ld: Warning: size of symbol
`_ZNSi6ignoreEv_at_GLIBCXX_3.4' changed from 243 in
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so to 211 in
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_ifstream<char, std::char_traits<char>
>::is_open() const'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::string::_M_copy(char*, char const*, unsigned long)'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::basic_ofstream<wchar_t, std::char_traits<wchar_t>
>::is_open() const'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::string::_M_check_length(unsigned long, unsigned
long, char const*) const'
/usr/obj/usr/src/tmp/usr/lib/libstdc++.so:(*IND*+0x0): multiple
definition of `std::string::_Rep::_M_set_length_and_sharable(unsigned long)'
clang++: error: linker command failed with exit code 1 (use -v to see
invocation)
*** [gperf] Error code 1

Stop in /usr/src/gnu/usr.bin/gperf.
*** [all] Error code 1

Stop in /usr/src/gnu/usr.bin.
*** [all] Error code 1

Stop in /usr/src/gnu.
*** [gnu.all__D] Error code 1

Stop in /usr/src.
*** [everything] Error code 1

Stop in /usr/src.
*** [buildworld] Error code 1

Stop in /usr/src.