patch 9.1.0393: 'viewdir' not respecting $XDG_CONFIG_HOME

Problem:  'viewdir' not respecting $XDG_CONFIG_HOME
          (Danilo Rezende, after v9.1.327)
Solution: adjust 'viewdir' option when enabling XDG config mode

fixes: #14680
closes: #14708

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/os_unix.h b/src/os_unix.h
index 6efd8ce..99184ab 100644
--- a/src/os_unix.h
+++ b/src/os_unix.h
@@ -347,6 +347,8 @@
 #  define DFLT_VDIR    "sys$login:vimfiles/view"
 # else
 #  define DFLT_VDIR    "$HOME/.vim/view"       // default for 'viewdir'
+#  define XDG_VDIR     (mch_getenv("XDG_CONFIG_HOME") ? \
+	"$XDG_CONFIG_HOME/vim/view" : "~/.config/vim/view")
 # endif
 #endif