libio build fails with glibc 2.1.94 installed

[Joseph S. Myers]

On i686-pc-linux-gnu with glibc 2.1.94 installed, a bootstrap of current
GCC CVS fails in libio:

/home/jsm28/src/gcc-build/gcc/g++ -B/home/jsm28/src/gcc-build/gcc/ \
 -nostdinc++ -isystem /home/jsm28/src/gcc-cvs/libstdc++ -isystem \
 /home/jsm28/src/gcc-cvs/libstdc++/std -isystem \
 /home/jsm28/src/gcc-cvs/libstdc++/stl -isystem \
 /home/jsm28/src/gcc-cvs/libio -isystem \
 /home/jsm28/src/gcc-build/i686-pc-linux-gnu/libio \
 -L/home/jsm28/src/gcc-build/i686-pc-linux-gnu/libstdc++ \
 -B/opt/gcc/snapshot/i686-pc-linux-gnu/bin/ \
 -B/opt/gcc/snapshot/i686-pc-linux-gnu/lib/ -isystem \
 /opt/gcc/snapshot/i686-pc-linux-gnu/include -c -g -O2 -fvtable-thunks \
 -D_GNU_SOURCE -fno-implicit-templates -I. -I../../../gcc-cvs/libio \
 -nostdinc++ -D_IO_MTSAFE_IO ../../../gcc-cvs/libio/iostream.cc
../../../gcc-cvs/libio/iostream.cc: In member function `istream &istream::get 
   (char &)':
../../../gcc-cvs/libio/iostream.cc:74: `_IO_USER_LOCK' undeclared (first use
   this function)
../../../gcc-cvs/libio/iostream.cc:74: (Each undeclared identifier is reported
   only once for each function it appears in.)
make[2]: *** [iostream.o] Error 1
make[2]: Leaving directory `/home/jsm28/src/gcc-build/i686-pc-linux-gnu/libio'
make[1]: *** [all-target-libio] Error 2
make[1]: Leaving directory `/home/jsm28/src/gcc-build'
make: *** [bootstrap] Error 2

Configuration: ../gcc-cvs/configure --prefix=/opt/gcc/snapshot \
 --disable-shared --enable-threads=posix --with-system-zlib

-- 
Joseph S. Myers
jsm28@cam.ac.uk