Do not attempt to outguess system headers by (re)defining errno.

Do not attempt to outguess system headers by (re)defining errno.

Recent glibc define error as a cpp macro expanding to a function
call.  Thus, "extern int errno" should cause a link-time undefined
symbol when errno.h is not included.  Here it is, and gcc complains
with the following cryptic warning:

../../gnushogi/sysdeps.c: In function 'ElapsedTime_FIONREAD':
../../gnushogi/sysdeps.c:97: warning: function declaration isn't a prototype