Integration of this synctex commit has been requested upstream:

  <https://bugs.launchpad.net/qpdfview/+bug/2006446>

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`.

diff --git a/synctex/synctex_parser.c b/synctex/synctex_parser.c
index 45124ea3bff536ef..cfb637709dfbfe2e 100644
--- a/synctex/synctex_parser.c
+++ b/synctex/synctex_parser.c
@@ -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