patch 9.1.0327: No support for using $XDG_CONFIG_HOME

Problem:  No support for using $XDG_CONFIG_HOME
Solution: optionally source $XDG_CONFIG_HOME/vim/vimrc
          (Luca Saccarola)

fixes: #2034
closes: #14182

Signed-off-by: Luca Saccarola <github.e41mv@aleeas.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/main.c b/src/main.c
index 5a41a1a..e5faaa7 100644
--- a/src/main.c
+++ b/src/main.c
@@ -3276,6 +3276,10 @@
 		&& do_source((char_u *)USR_VIMRC_FILE2, TRUE,
 						      DOSO_VIMRC, NULL) == FAIL
 #endif
+#ifdef XDG_VIMRC_FILE
+		&& do_source((char_u *)XDG_VIMRC_FILE, TRUE,
+						      DOSO_VIMRC, NULL) == FAIL
+#endif
 #ifdef USR_VIMRC_FILE3
 		&& do_source((char_u *)USR_VIMRC_FILE3, TRUE,
 						      DOSO_VIMRC, NULL) == FAIL