post_install.sh: create ags symlink relative to Root directory (#503)

Creating the ags executable as an absolute symlink to
$DESTDIR/$2/$APP_ID breaks staged installs, where the installed files
are not placed directly into their expected location but are instead
copied into a temporary location (DESTDIR).
Since installed files maintain their relative directory structure and
any embedded file names will not be modified, we can create a relative
symlink instead, which fixes the issue.

It has the added benefit that the link target remains valid, even if the
storage device underlying this program is mounted into a subdirectory of
the root filesystem (e.g. for recovery).

Bug: gentoo/guru#212

post_install.sh

@@ -9,7 +9,7 @@ mkdir -p $BIN_DIR
 
 BIN_SRC="$PKGDATA_DIR/$APP_ID"
 BIN_DEST="$BIN_DIR/ags"
-ln -s -f $BIN_SRC $BIN_DEST
+ln -s -r -f $BIN_SRC $BIN_DEST
 
 if [[ "$5" == "false" ]]; then
 	exit 0