Improve broken shell warning.

Originally committed as revision 7278 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -40,6 +40,9 @@ if test "$E1" != 0 || test "$E2" = 0; then
     echo "No compatible shell script interpreter found."
     echo "This configure script requires a POSIX compatible shell"
     echo "such as bash or ksh."
+    echo "THIS IS NOT A BUG IN FFMPEG, DO NOT REPORT IT AS SUCH."
+    echo "Instead, install a working POSIX-compatible shell."
+    echo "Disabling this configure test will create a broken FFmpeg."
     if test "$BASH_VERSION" = '2.04.0(1)-release'; then
         echo "This bash version ($BASH_VERSION) is broken on your platform."
         echo "Upgrade to a later version if available."