diff --git a/linuxdoc-tools-c99.patch b/linuxdoc-tools-c99.patch
new file mode 100644
index 0000000..fe875bf
--- /dev/null
+++ b/linuxdoc-tools-c99.patch
@@ -0,0 +1,296 @@
+From 0771d5d9a49ccd2e4c10ce1fe9bb14b7f69d4081 Mon Sep 17 00:00:00 2001
+From: Florian Weimer <fweimer@redhat.com>
+Date: Thu, 13 Apr 2023 18:09:27 +0200
+Subject: [PATCH] sgmls: Avoid implicit ints/function declarations in configure
+Content-type: text/plain
+
+These C features are no longer part of C99, and future compilers
+will no longer support them by default, causing the build to fail.
+
+The changes assume that the usual system headers (<stdlib.h>,
+<unistd.h>) are present, but that should not be a problem on current
+systems.
+
+Submitted upstream: <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1034362>
+
+---
+ sgmls-1.1/configure | 60 +++++++++++++++++++++++++++++++--------------
+ 1 file changed, 42 insertions(+), 18 deletions(-)
+
+diff --git a/sgmls-1.1/configure b/sgmls-1.1/configure
+index 5d3e197..e674d24 100755
+--- a/sgmls-1.1/configure
++++ b/sgmls-1.1/configure
+@@ -110,13 +110,15 @@ cat >doit.c <<\EOF
+ 
+ #include <ctype.h>
+ #include <signal.h>
++#include <stdlib.h>
++#include <unistd.h>
+ 
+ static int whoops()
+ {
+   _exit(1);
+ }
+ 
+-main()
++int main(void)
+ {
+   int c;
+ #ifdef isascii
+@@ -213,13 +215,15 @@ else
+ fi
+ 
+ cat >doit.c <<\EOF
++#include <stdio.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+ {
+   if (argc == 0)
+     remove("foo");
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -231,13 +235,15 @@ else
+ fi
+ 
+ cat >doit.c <<\EOF
++#include <unistd.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+ {
+   if (argc == 0)
+     getopt(argc, argv, "v");
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -248,14 +254,16 @@ else
+ 	off="$off HAVE_GETOPT"
+ fi
+ 
+-cat >doit.c <<\EOF
++cat>doit.c <<\EOF
++#include <unistd.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+ {
+   if (argc == 0)
+     access("foo", 4);
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -267,13 +275,15 @@ else
+ fi
+ 
+ cat >doit.c <<\EOF
++#include <unistd.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+ {
+   if (argc == 0)
+     vfork();
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -285,6 +295,8 @@ else
+ fi
+ 
+ cat >doit.c <<\EOF
++#include <sys/wait.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+@@ -294,7 +306,7 @@ char **argv;
+     int status;
+     waitpid(-1, &status, 0);
+   }
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -307,13 +319,14 @@ fi
+ 
+ cat >doit.c <<\EOF
+ #include <string.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+ {
+   if (argc == 0)
+     strerror(0);
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -326,13 +339,14 @@ fi
+ 
+ cat >doit.c <<\EOF
+ #include <strings.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+ {
+   if (argc == 0)
+ 	bcopy((char *)0, (char *)0, 0);
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -341,13 +355,14 @@ then
+ 	# Only use BSD_STRINGS if ANSI string functions don't work.
+ 	cat >doit.c <<\EOF
+ #include <string.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+ {
+   if (argc == 0)
+ 	memcpy((char *)0, (char *)0, 0);
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -363,13 +378,14 @@ fi
+ 
+ cat >doit.c <<\EOF
+ #include <signal.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+ {
+   if (argc == 0)
+     raise(SIGINT);
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -382,6 +398,7 @@ fi
+ 
+ cat >doit.c <<\EOF
+ #include <stdio.h>
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+@@ -391,7 +408,7 @@ char **argv;
+     fsetpos(stdin, &pos);
+     fgetpos(stdin, &pos);
+   }
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -407,6 +424,7 @@ cat >doit.c <<\EOF
+ #include <sys/types.h>
+ #include <sys/wait.h>
+ 
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+@@ -423,7 +441,7 @@ char **argv;
+     WTERMSIG(status);
+     WSTOPSIG(status);
+   }
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -437,13 +455,16 @@ fi
+ cat >doit.c <<\EOF
+ #include <stdio.h>
+ #include <signal.h>
++#include <stdlib.h>
++#include <unistd.h>
++#include <string.h>
+ 
+ static int whoops()
+ {
+   _exit(1);
+ }
+ 
+-main()
++int main(void)
+ {
+   char buf[30];
+ #ifdef SIGSEGV
+@@ -470,6 +491,7 @@ fi
+ cat >doit.c <<\EOF
+ #include <nl_types.h>
+ 
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+@@ -479,7 +501,7 @@ char **argv;
+     catgets(d, 1, 1, "default");
+     catclose(d);
+   }
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+@@ -492,9 +514,11 @@ fi
+ 
+ cat >doit.c <<\EOF
+ #include <limits.h>
++#include <stdlib.h>
+ 
+ char c = UCHAR_MAX;
+ 
++int
+ main(argc, argv)
+ int argc;
+ char **argv;
+@@ -512,16 +536,16 @@ then
+ 	char_signed=
+ else
+ 	cat >doit.c <<\EOF
+-main()
++int main(void)
+ {
+   int i;
+ 
+   for (i = 0; i < 512; i++) {
+     char c = (char)i;
+     if (c < 0)
+-	exit(1);
++	return 1;
+   }
+-  exit(0);
++  return 0;
+ }
+ EOF
+ 
+-- 
+2.39.2
+
diff --git a/linuxdoc-tools.spec b/linuxdoc-tools.spec
index bf521e5..b8fb682 100644
--- a/linuxdoc-tools.spec
+++ b/linuxdoc-tools.spec
@@ -5,11 +5,12 @@
 Summary: A text formatting package based on SGML
 Name: linuxdoc-tools
 Version: 0.9.82
-Release: 1%{?dist}
+Release: 2%{?dist}
 License: MIT
 Source: http://http.us.debian.org/debian/pool/main/l/linuxdoc-tools/%{name}_%{version}.orig.tar.gz
 Patch01: 0001-downstream-Changed-default-papersize-to-letter.patch
 Patch02: 0002-downstream-Added-fix-to-have-lib64-in-perl-path-on-6.patch
+Patch3: linuxdoc-tools-c99.patch
 Url: http://packages.qa.debian.org/l/linuxdoc-tools.html
 BuildRequires: git gcc
 BuildRequires: flex flex-static sgml-common jade gawk groff autoconf automake texinfo
@@ -102,6 +103,9 @@ exit 0
 %{_mandir}/*/*
 
 %changelog
+* Thu Apr 13 2023 Florian Weimer <fweimer@redhat.com> - 0.9.82-2
+- Port sgmls non-autoconf configure script to C99
+
 * Sun Feb 05 2023 Julien Rische <jrische@redhat.com> - 0.9.82-1
 - New version 0.9.82