Check for the isatty function.

Patch by Martin Storsjo <martin at martin dot st>

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@19504 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -925,6 +925,7 @@ HAVE_LIST="
     getrusage
     inet_aton
     inline_asm
+    isatty
     ldbrx
     libdc1394_1
     libdc1394_2
@@ -2118,6 +2119,7 @@ check_func  fork
 check_func  gethrtime
 check_func  getrusage
 check_func  inet_aton $network_extralibs
+check_func  isatty
 check_func  memalign
 check_func  mkstemp
 check_func  posix_memalign