build: Do not replace existing files

The existing INSTALL file is currently being replaced at every
autogen.sh run with the default one.

This is not desired, because INSTALL contains some cairo-specific
information.

Author: ranma42

autogen.sh

@@ -25,7 +25,7 @@ fi
 > src/Makefile.am.features
 touch ChangeLog
 
-autoreconf --force --install --verbose || exit $?
+autoreconf --install --verbose || exit $?
 
 cd $ORIGDIR
 test -n "$NOCONFIGURE" || "$srcdir/configure" "$@"