configure: check for unistd.h

Signed-off-by: NMans Rullgard <mans@mansr.com>

configure

@@ -1150,6 +1150,7 @@ HAVE_LIST="
     threads
     trunc
     truncf
+    unistd_h
     vfp_args
     VirtualAlloc
     winsock2_h
@@ -2866,6 +2867,7 @@ check_header sys/mman.h
 check_header sys/param.h
 check_header sys/resource.h
 check_header sys/select.h
+check_header unistd.h
 check_header vdpau/vdpau.h
 check_header vdpau/vdpau_x11.h
 check_header X11/extensions/XvMClib.h