Don't hardcode the shlibdir on Cygwin, it's supposed to be bindir.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@7222 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -746,7 +746,7 @@ mingw32="yes"
 ;;
 CYGWIN*)
 targetos=CYGWIN
-shlibdir='${PREFIX}/bin'
+shlibdir="$bindir"
 video4linux="no"
 video4linux2="no"
 audio_oss="yes"