Partial backport of this upstream revision:

------------------------------------------------------------------------
r65582 | hironobu | 2023-01-19 15:15:46 +0100 (Thu, 19 Jan 2023) | 1 line

synctexdir: update from GH

Relevant Git commit:

commit 2897465154892a7737dcc90e4d6a00a1d1b3922c
Author: Germán Poo-Caamaño <gpoo@gnome.org>
Date:   Sat May 30 16:15:27 2020 -0400

    Fix implicit declaration of vasprintf
    
    Fix build when compiling with `-Werror=implicit-function-declaration`.


Index: Build/source/texk/web2c/synctexdir/synctex_parser.c
===================================================================
--- Build/source/texk/web2c/synctexdir/synctex_parser.c	(revision 65581)
+++ Build/source/texk/web2c/synctexdir/synctex_parser.c	(revision 65582)
@@ -83,6 +83,11 @@
  *  First level objects are sheets and forms, containing boxes, glues, kerns...
  *  The third tree allows to browse leaves according to tag and line.
  */
+/* Declare _GNU_SOURCE for accessing vasprintf. For MSC compiler, vasprintf is
+ * defined in this file
+ */
+#define _GNU_SOURCE
+
 #   if defined(SYNCTEX_USE_LOCAL_HEADER)
 #       include "synctex_parser_local.h"
 #   else