diff --git a/clearsilver-configure-c99.patch b/clearsilver-configure-c99.patch
new file mode 100644
index 0000000..7b12e89
--- /dev/null
+++ b/clearsilver-configure-c99.patch
@@ -0,0 +1,42 @@
+Avoid implicit function declarations in the configure script.  This
+helps to ensure compatibility with future compilers, which are likely
+to reject them by default.
+
+In the unlikely event of another upstream release, current autoconf no
+longer produces these script fragments, so the issue should go away on
+its own.
+
+diff --git a/configure b/configure
+index e49342eb47e20ca1..9a331621d4059d1c 100755
+--- a/configure
++++ b/configure
+@@ -2124,7 +2124,7 @@ if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
+   echo "$as_me:$LINENO: \$? = $ac_status" >&5
+   (exit $ac_status); }; }; then
+   for ac_declaration in \
+-   '' \
++   '#include <stdlib.h>' \
+    'extern "C" void std::exit (int) throw (); using std::exit;' \
+    'extern "C" void std::exit (int); using std::exit;' \
+    'extern "C" void exit (int) throw ();' \
+@@ -3221,8 +3221,8 @@ main ()
+   for (i = 0; i < 256; i++)
+     if (XOR (islower (i), ISLOWER (i))
+ 	|| toupper (i) != TOUPPER (i))
+-      exit(2);
+-  exit (0);
++      return 2;
++  return 0;
+ }
+ _ACEOF
+ rm -f conftest$ac_exeext
+@@ -4697,6 +4697,9 @@ cat >>conftest.$ac_ext <<_ACEOF
+ #include <sys/time.h>
+ #include <sys/resource.h>
+ #include <stdio.h>
++#include <unistd.h>
++#include <sys/wait.h>
++#include <stdlib.h>
+ /* HP-UX has wait3 but does not fill in rusage at all.  */
+ int
+ main ()
diff --git a/clearsilver.spec b/clearsilver.spec
index 74cb75a..7f0863b 100644
--- a/clearsilver.spec
+++ b/clearsilver.spec
@@ -2,7 +2,7 @@
 
 Name:           clearsilver
 Version:        0.10.5
-Release:        69%{?dist}
+Release:        70%{?dist}
 Summary:        Fast and powerful HTML templating system
 # Technically, the license is "Neotonic ClearSilver", but it is a copy of 
 # ASL 1.1 with the trademarks as the only difference.
@@ -16,6 +16,7 @@ Patch3:		clearsilver-ruby-1.9.patch
 Patch4:		clearsilver-ruby-2.2.patch
 # GCC 5 compatibility, bug #1190760
 Patch5:		clearsilver-0.10.5-gcc5.patch
+Patch6:		clearsilver-configure-c99.patch
 BuildRequires:  gcc
 BuildRequires:  zlib-devel
 BuildRequires:  httpd-devel
@@ -84,6 +85,7 @@ Provides:       %{name}-java = %{version}-%{release}
 %patch3 -p1
 %patch4 -p1
 %patch5 -p2
+%patch6 -p1
 touch configure
 sed -i -r 's|(\$\(RUBY\) install.rb config) (--.*)|\1 --rb-dir="$(DESTDIR)%{ruby_vendorlibdir}" --so-dir="$(DESTDIR)%{ruby_vendorarchdir}" \2|' ruby/Makefile
 
@@ -157,6 +159,9 @@ ln -s clearsilver-%{version}.jar $RPM_BUILD_ROOT%{_libdir}/java/clearsilver.jar
 
 
 %changelog
+* Mon Dec 12 2022 Florian Weimer <fweimer@redhat.com> - 0.10.5-70
+- Port configure script to C99
+
 * Wed Jul 20 2022 Fedora Release Engineering <releng@fedoraproject.org> - 0.10.5-69
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_37_Mass_Rebuild