xmlto: fix build with clang 16

Fix implicit `int` on `main` and `static` declarations. Clang 16 reports
these as errors.

pkgs/tools/typesetting/xmlto/default.nix

@@ -1,5 +1,5 @@
 { fetchurl, lib, stdenv, libxml2, libxslt
-, docbook_xml_dtd_45, docbook_xsl, w3m
+, docbook_xml_dtd_45, docbook_xsl, flex, w3m
 , bash, getopt, makeWrapper }:
 
 stdenv.mkDerivation rec {
@@ -10,6 +10,9 @@ stdenv.mkDerivation rec {
     sha256 = "0xhj8b2pwp4vhl9y16v3dpxpsakkflfamr191mprzsspg4xdyc0i";
   };
 
+  # Implicit `int` is an error with clang 16.
+  patches = [ ./fix-implicit-int.patch ];
+
   postPatch = ''
     patchShebangs xmlif/test/run-test
 
@@ -23,7 +26,7 @@ stdenv.mkDerivation rec {
 
   # `libxml2' provides `xmllint', needed at build-time and run-time.
   # `libxslt' provides `xsltproc', used by `xmlto' at run-time.
-  nativeBuildInputs = [ makeWrapper getopt ];
+  nativeBuildInputs = [ makeWrapper flex getopt ];
   buildInputs = [ libxml2 libxslt docbook_xml_dtd_45 docbook_xsl ];
 
   postInstall = ''

pkgs/tools/typesetting/xmlto/fix-implicit-int.patch

@@ -0,0 +1,21 @@
+diff -ur a/xmlif/xmlif.l b/xmlif/xmlif.l
+--- a/xmlif/xmlif.l	2015-02-11 07:20:51.000000000 -0500
++++ b/xmlif/xmlif.l	2023-06-03 11:35:11.394139373 -0400
+@@ -43,7 +43,7 @@
+
+ static char **selections;	/* selection tokens */
+ static int nselections;		/* number of selections */
+-static ifsense;			/* sense of last `if' or unless seen */
++static int ifsense;		/* sense of last `if' or unless seen */
+ static char *attribute;		/* last attribute scanned */
+
+ struct stack_t {
+@@ -239,7 +239,7 @@
+
+ int yywrap() {exit(0);};
+
+-main(int argc, char *argv[])
++int main(int argc, char *argv[])
+ {
+     int i;
+