Simplify source_path handling, avoids some warnings with non-bash shells.

Originally committed as revision 4807 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -416,7 +416,7 @@ TARGET_OS=`( uname -s ) 2>&1`
   esac
 
 # find source path
-source_path="`echo $0 | sed -e 's#/configure##'`"
+source_path="`dirname $0`"
 source_path_used="yes"
 if test -z "$source_path" -o "$source_path" = "." ; then
     source_path=`pwd`