FTBFS with newer glibc 2.27

Newer rebuilds of formerly working code show the following:

cc -I/<<BUILDDIR>>/qemu-2.11+dfsg/qemu-build/util -Iutil -I/<<BUILDDIR>>/qemu-2.11+dfsg/tcg -I/<<BUILDDIR>>/qemu-2.11+dfsg/tcg/i386 -I/<<BUILDDIR>>/qemu-2.11+dfsg/linux-headers -I/<<BUILDDIR>>/qemu-2.11+dfsg/qemu-build/linux-headers -I. -I/<<BUILDDIR>>/qemu-2.11+dfsg -I/<<BUILDDIR>>/qemu-2.11+dfsg/accel/tcg -I/<<BUILDDIR>>/qemu-2.11+dfsg/include -I/usr/include/pixman-1 -pthread -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -pthread -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -DNCURSES_WIDECHAR -D_GNU_SOURCE -D_DEFAULT_SOURCE -I/usr/include/ncursesw -fPIE -DPIE -m64 -mcx16 -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -Wstrict-prototypes -Wredundant-decls -Wall -Wundef -Wwrite-strings -Wmissing-prototypes -fno-strict-aliasing -fno-common -fwrapv -g -O2 -fdebug-prefix-map=/<<BUILDDIR>>/qemu-2.11+dfsg=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -DCONFIG_QEMU_DATAPATH='"/usr/share/qemu:/usr/share/seabios:/usr/lib/ipxe/qemu"' -DVENDOR_UBUNTU -Wexpansion-to-defined -Wendif-labels -Wno-shift-negative-value -Wno-missing-include-dirs -Wempty-body -Wnested-externs -Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers -Wold-style-declaration -Wold-style-definition -Wtype-limits -fstack-protector-strong -I/usr/include/p11-kit-1 -I/usr/include/libpng16 -I/usr/include/spice-server -I/usr/include/spice-1 -I/<<BUILDDIR>>/qemu-2.11+dfsg/tests -I qga/qapi-generated -MMD -MP -MT util/memfd.o -MF util/memfd.d -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -g -c -o util/memfd.o /<<BUILDDIR>>/qemu-2.11+dfsg/util/memfd.c
cc -I/<<BUILDDIR>>/qemu-2.11+dfsg/qemu-build/util -Iutil -I/<<BUILDDIR>>/qemu-2.11+dfsg/tcg -I/<<BUILDDIR>>/qemu-2.11+dfsg/tcg/i386 -I/<<BUILDDIR>>/qemu-2.11+dfsg/linux-headers -I/<<BUILDDIR>>/qemu-2.11+dfsg/qemu-build/linux-headers -I. -I/<<BUILDDIR>>/qemu-2.11+dfsg -I/<<BUILDDIR>>/qemu-2.11+dfsg/accel/tcg -I/<<BUILDDIR>>/qemu-2.11+dfsg/include -I/usr/include/pixman-1 -pthread -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -pthread -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -DNCURSES_WIDECHAR -D_GNU_SOURCE -D_DEFAULT_SOURCE -I/usr/include/ncursesw -fPIE -DPIE -m64 -mcx16 -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -Wstrict-prototypes -Wredundant-decls -Wall -Wundef -Wwrite-strings -Wmissing-prototypes -fno-strict-aliasing -fno-common -fwrapv -g -O2 -fdebug-prefix-map=/<<BUILDDIR>>/qemu-2.11+dfsg=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -DCONFIG_QEMU_DATAPATH='"/usr/share/qemu:/usr/share/seabios:/usr/lib/ipxe/qemu"' -DVENDOR_UBUNTU -Wexpansion-to-defined -Wendif-labels -Wno-shift-negative-value -Wno-missing-include-dirs -Wempty-body -Wnested-externs -Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers -Wold-style-declaration -Wold-style-definition -Wtype-limits -fstack-protector-strong -I/usr/include/p11-kit-1 -I/usr/include/libpng16 -I/usr/include/spice-server -I/usr/include/spice-1 -I/<<BUILDDIR>>/qemu-2.11+dfsg/tests -I qga/qapi-generated -MMD -MP -MT util/envlist.o -MF util/envlist.d -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -g -c -o util/envlist.o /<<BUILDDIR>>/qemu-2.11+dfsg/util/envlist.c
/<<BUILDDIR>>/qemu-2.11+dfsg/util/memfd.c:40:12: error: static declaration of ‘memfd_create’ follows non-static declaration
 static int memfd_create(const char *name, unsigned int flags)
            ^~~~~~~~~~~~
In file included from /usr/include/x86_64-linux-gnu/bits/mman-linux.h:115:0,
                 from /usr/include/x86_64-linux-gnu/bits/mman.h:45,
                 from /usr/include/x86_64-linux-gnu/sys/mman.h:41,
                 from /<<BUILDDIR>>/qemu-2.11+dfsg/include/sysemu/os-posix.h:29,
                 from /<<BUILDDIR>>/qemu-2.11+dfsg/include/qemu/osdep.h:104,
                 from /<<BUILDDIR>>/qemu-2.11+dfsg/util/memfd.c:28:
/usr/include/x86_64-linux-gnu/bits/mman-shared.h:46:5: note: previous declaration of ‘memfd_create’ was here
 int memfd_create (const char *__name, unsigned int __flags) __THROW;
     ^~~~~~~~~~~~
/<<BUILDDIR>>/qemu-2.11+dfsg/rules.mak:66: recipe for target 'util/memfd.o' failed
make[1]: *** [util/memfd.o] Error 1
make[1]: *** Waiting for unfinished jobs....