strchrnul() is a GNU extension

Index: gschem/src/x_controlfd.c
--- gschem/src/x_controlfd.c.orig
+++ gschem/src/x_controlfd.c
@@ -195,10 +195,11 @@ split_args (gchar ***args_return, gint *count_return, 
   GSList *tokens = NULL;
   size_t len = strlen (buf);
   size_t skip = 0;
+  gchar *p;
 
   do {
-    size_t sp = strchrnul (buf + skip, ' ') - buf;
-    size_t bs = strchrnul (buf + skip, '\\') - buf;
+    size_t sp = (p = strchr (buf + skip, ' ')) != NULL ? p - buf : len;
+    size_t bs = (p = strchr (buf + skip, '\\')) != NULL ? p - buf : len;
 
     if (sp < bs) {
       if (sp != 0)