I tried to build gdbserver as host=i686-mingw32 on linux, but get the
following error,

cc1: warnings being treated as errors
../gdb/gdb/gdbserver/hostio.c: In function 'handle_readlink':
../gdb/gdb/gdbserver/hostio.c:475: error: implicit declaration of
function 'readlink'

Even I remove -Werror temporarily, `readlink' can't be found during
linking,

hostio.o: In function `handle_readlink':
../gdb/gdb/gdbserver/hostio.c:475: undefined reference to `_readlink'

This patch is to add `readlink' in AC_CHECK_FUNCS, to check it
during configure.  OK to apply?

-- 
Yao (齐尧)