Fix error.h to include errno.h for glibc >= 2.3.x

Refer to the following (look for errno):
  http://thedjbway.b0llix.net/errno.html
  http://qmail.org/top.html
  http://cr.yp.to/docs/unixport.html

Just one command can fix all such for most DJB software:

Or, djb suggests that you edit conf-cc to read:

cc -O2 -include /usr/include/errno.h

Signed-off-by: Giam Teck Choon <giamteckchoon@gmail.com>