Build failure with latest ICU 64.1

The rebuild of LibreOffice after upgrade to most recent ICU 64.1 fails as follows (also reproduced 
on a different machine with 6.2.2.2):

[build CXX] xmlsecurity/source/xmlsec/biginteger.cxx
S=/var/tmp/portage/app-office/libreoffice-6.2.1.2/work/libreoffice-6.2.1.2 && I=$S/instdir && 
W=$S/workdir &&  mkdir -p $W/CxxObject/xmlsecurity/source/xmlsec/ 
$W/Dep/CxxObject/xmlsecurity/source/xmlsec/ && cd 
/var/tmp/portage/app-office/libreoffice-6.2.1.2/work/libreoffice-6.2.1.2 &&    
x86_64-pc-linux-gnu-g++ -DBOOST_ERROR_CODE_HEADER_ONLY -DBOOST_SYSTEM_NO_DEPRECATED -DCPPU_ENV=gcc3 
-DLINUX -DNDEBUG -DOSL_DEBUG_LEVEL=0 -DUNIX -DUNX -DX86_64 -D_PTHREADS -D_REENTRANT   
-DXMLSEC_NO_XSLT -DXSECXMLSEC_DLLIMPLEMENTATION -DXSECGPG_DLLIMPLEMENTATION  -DSYSTEM_LIBXML  
-DSYSTEM_XMLSEC  -DXMLSEC_CRYPTO_NSS  -DSYSTEM_NSS  -DSYSTEM_NSS   -fvisibility=hidden    -Wall 
-Wno-missing-braces -Wnon-virtual-dtor -Wendif-labels -Wextra -Wundef -Wunreachable-code 
-Wunused-macros -finput-charset=UTF-8 -fmessage-length=0 -fno-common -pipe  -Wduplicated-cond 
-Wlogical-op -Wshift-overflow=2 -Wunused-const-variable=1 -Wno-cast-function-type 
-fvisibility-inlines-hidden -fstack-protector-strong -fPIC -Wshadow -Woverloaded-virtual 
-std=gnu++2a   -DEXCEPTIONS_ON -fexceptions -fno-enforce-eh-specs -march=native -mtune=native -O2 
-pipe   -DLIBO_INTERNAL_ONLY  -c $S/xmlsecurity/source/xmlsec/biginteger.cxx -o 
$W/CxxObject/xmlsecurity/source/xmlsec/biginteger.o  -I$S/include  -I/usr/lib64/icedtea8/include 
-I/usr/lib64/icedtea8/include/linux -I$S/config_host  -I$S/xmlsecurity/inc 
-I$S/xmlsecurity/source/gpg -I$S/xmlsecurity/source/xmlsec -I$W/UnpackedTarball/xmlsec/include  
-I$W/CustomTarget/officecfg/registry -I$W/UnoApiHeadersTarget/udkapi/normal 
-I$W/UnoApiHeadersTarget/offapi/normal -I/usr/include   -I/usr/include/gpgme++   -isystem 
/usr/include/libxml2   -DXMLSEC_CRYPTO_NSS=1 -D__XMLSEC_FUNCTION__=__func__ -DXMLSEC_NO_SIZE_T 
-DXMLSEC_NO_AES=1 -DXMLSEC_NO_GOST=1 -DXMLSEC_NO_GOST2012=1 -DXMLSEC_DL_LIBLTDL=1 -isystem 
/usr/include/xmlsec1 -isystem /usr/include/libxml2 -isystem /usr/include/nss -isystem 
/usr/include/nspr   -isystem /usr/include/nss -isystem /usr/include/nspr   -isystem 
/usr/include/nss -isystem /usr/include/nspr    
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/include/g++-v8/memory:64,
                 from /usr/include/unicode/localpointer.h:45,
                 from /usr/include/unicode/uenum.h:23,
                 from /usr/include/unicode/ucnv.h:53,
                 from /usr/include/libxml2/libxml/encoding.h:31,
                 from /usr/include/libxml2/libxml/parser.h:810,
                 from /usr/include/libxml2/libxml/globals.h:18,
                 from /usr/include/libxml2/libxml/threads.h:35,
                 from /usr/include/libxml2/libxml/xmlmemory.h:218,
                 from /usr/include/libxml2/libxml/tree.h:1307,
                 from /usr/include/xmlsec1/xmlsec/base64.h:18,
                 from 
/var/tmp/portage/app-office/libreoffice-6.2.1.2/work/libreoffice-6.2.1.2/xmlsecurity/inc/xmlsec-wrapper.h:32,
                 from 
/var/tmp/portage/app-office/libreoffice-6.2.1.2/work/libreoffice-6.2.1.2/xmlsecurity/source/xmlsec/biginteger.cxx:23:
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/include/g++-v8/bits/stl_construct.h:72:3: error: template 
with C linkage
   template<typename _T1, typename... _Args>
   ^~~~~~~~
In file included from 
/var/tmp/portage/app-office/libreoffice-6.2.1.2/work/libreoffice-6.2.1.2/xmlsecurity/inc/xmlsec-wrapper.h:32,
                 from 
/var/tmp/portage/app-office/libreoffice-6.2.1.2/work/libreoffice-6.2.1.2/xmlsecurity/source/xmlsec/biginteger.cxx:23:
/usr/include/xmlsec1/xmlsec/base64.h:15:1: note:  extern "C" {
 ^~~~~~~~~~
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/include/g++-v8/memory:64,
                 from /usr/include/unicode/localpointer.h:45,
                 from /usr/include/unicode/uenum.h:23,
                 from /usr/include/unicode/ucnv.h:53,
                 from /usr/include/libxml2/libxml/encoding.h:31,
                 from /usr/include/libxml2/libxml/parser.h:810,
                 from /usr/include/libxml2/libxml/globals.h:18,
                 from /usr/include/libxml2/libxml/threads.h:35,
                 from /usr/include/libxml2/libxml/xmlmemory.h:218,
                 from /usr/include/libxml2/libxml/tree.h:1307,
                 from /usr/include/xmlsec1/xmlsec/base64.h:18,
                 from 
/var/tmp/portage/app-office/libreoffice-6.2.1.2/work/libreoffice-6.2.1.2/xmlsecurity/inc/xmlsec-wrapper.h:32,
                 from 
/var/tmp/portage/app-office/libreoffice-6.2.1.2/work/libreoffice-6.2.1.2/xmlsecurity/source/xmlsec/biginteger.cxx:23:
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/include/g++-v8/bits/stl_construct.h:87:3: error: template 
with C linkage
   template<typename _T1>
   ^~~~~~~~

...and so on. Notably, this has been the only reverse-dependency failing with this ICU upgrade so 
far.

Full build.log (beware, ~14M): https://www.gentoofan.org/gentoo/misc/libreoffice-6.2.1.2-build.log

Best regards,
Andreas