Include <wctype.h> to avoid implicit function declaration of towupper.
Fixed in gnulib upstream via:

commit 2d548920742b7735c1d4722c87f59c1df3b06556
Author: Bruno Haible <bruno@clisp.org>
Date:   Sat Sep 14 19:41:46 2019 +0200

    Make autoconf tests work with -Werror=implicit-function-declaration.
    
    * m4/locale-tr.m4 (gt_LOCALE_TR_UTF8): Include <wctype.h>, for
    towupper() declaration.
    * m4/ptsname.m4 (gl_FUNC_PTSNAME): Include <stdlib.h>, for ptsname()
    declaration.

diff --git a/configure b/configure
index 505535b165bd8cfc..3f7015b78ab1bb99 100755
--- a/configure
+++ b/configure
@@ -36759,6 +36759,7 @@ else
 #endif
 #include <stdlib.h>
 #include <string.h>
+#include <wctype.h>
 struct tm t;
 char buf[16];
 int main () {
diff --git a/libmissing/m4/locale-tr.m4 b/libmissing/m4/locale-tr.m4
index 2f2cf47892d505a5..454c18858d2b00c1 100644
--- a/libmissing/m4/locale-tr.m4
+++ b/libmissing/m4/locale-tr.m4
@@ -21,6 +21,7 @@ changequote(,)dnl
 #endif
 #include <stdlib.h>
 #include <string.h>
+#include <wctype.h>
 struct tm t;
 char buf[16];
 int main () {