add more info about new configure features

Originally committed as revision 583 to svn://svn.ffmpeg.org/ffmpeg/trunk

INSTALL

@@ -2,6 +2,11 @@
 1) Type './configure' create the configuration (use './configure
 --help' to have the configure options). 
 
+'configure' can be launched from another directory than the ffmpeg
+sources to put the objects at that place. In that case, use an
+absolute path when launching 'configure',
+e.g. /ffmpegdir/ffmpeg/configure.
+
 2) Then type 'make' to build ffmpeg.
 
 3) Type 'make install' to install ffmpeg and ffserver in