diff --git a/Macaulay2-configure-c99.patch b/Macaulay2-configure-c99.patch
new file mode 100644
index 0000000..2cdc1c6
--- /dev/null
+++ b/Macaulay2-configure-c99.patch
@@ -0,0 +1,18 @@
+Avoid an implicit int, which was removed from the C99 revision of the
+C language. This prevents altering the outcome of the check with
+future compilers.
+
+Submitted upstream: <https://github.com/Macaulay2/M2/pull/2814>
+
+diff -ur M2-release-1.21.orig/M2/configure.ac M2-release-1.21/M2/configure.ac
+--- M2-release-1.21.orig/M2/configure.ac	2023-04-15 19:52:33.823153026 +0200
++++ M2-release-1.21/M2/configure.ac	2023-04-15 20:00:57.701024623 +0200
+@@ -1719,7 +1719,7 @@
+     #ifdef HAVE_NETDB_H
+      #include <netdb.h>
+     #endif
+-    main() { 
++    int main(void) {
+        struct addrinfo *addr; 
+        return 0 != getaddrinfo("1.2.3.4", "80", 0, &addr) ? 99 : 0 ;
+        }
diff --git a/Macaulay2.spec b/Macaulay2.spec
index 7169b36..5f72200 100644
--- a/Macaulay2.spec
+++ b/Macaulay2.spec
@@ -52,7 +52,7 @@
 Summary: System for algebraic geometry and commutative algebra
 Name:    Macaulay2
 Version: 1.21
-Release: 4%{?dist}
+Release: 5%{?dist}
 
 # GPL-2.0-only OR GPL-3.0-only:
 #   - the project as a whole
@@ -264,6 +264,7 @@ Patch5: %{name}-1.17-configure.patch
 Patch6: %{name}-1.16-rightarrow.patch
 # Fix LTO warnings about mismatched declarations and definitions
 Patch7: %{name}-1.18-lto.patch
+Patch8: Macaulay2-configure-c99.patch
 
 BuildRequires: 4ti2
 BuildRequires: appstream
@@ -445,6 +446,7 @@ ln -s %{_datadir}/factory \
 %patch5 -p1 -b .configure
 %patch6 -p1 -b .rightarrow
 %patch7 -p1 -b .lto
+%patch8 -p2 -b .configure-c99
 
 # repeatable builds: inject a node name
 sed -i 's,`uname -n`,build.fedoraproject.org,' configure.ac
@@ -613,6 +615,9 @@ make check -C BUILD/%{_target_platform}/Macaulay2/bin
 
 
 %changelog
+* Sat Apr 15 2023 Florian Weimer <fweimer@redhat.com> - 1.21-5
+- Port configure script to C99
+
 * Mon Feb 20 2023 Jonathan Wakely <jwakely@redhat.com> - 1.21-4
 - Rebuilt for Boost 1.81