Robustify autogen.sh
Robustify autogen.sh

This allows the script to be run from somewhere other than the
 top-level build directory.

Signed-off-by: Erik de Castro Lopo <1586a8e151fb6d548e7a343b9bcdfea42e83ffc0@mega-nerd.com>

fredericgermain/flac,fredericgermain/flac,waitman/flac,fredericgermain/flac,Distrotech/flac,LordJZ/libflac,waitman/flac,Distrotech/flac,waitman/flac,LordJZ/libflac,Distrotech/flac,fredericgermain/flac,waitman/flac,LordJZ/libflac,Distrotech/flac,LordJZ/libflac,waitman/flac