diff --git a/plotutils-configure-c99.patch b/plotutils-configure-c99.patch
new file mode 100644
index 0000000..2825960
--- /dev/null
+++ b/plotutils-configure-c99.patch
@@ -0,0 +1,54 @@
+Do not call the undeclared exit function.  This avoids build problems
+with future compilers which do not support implicit function
+declarations.
+
+diff --git a/configure b/configure
+index 229ccd453ae4487f..fdf3785ad4c5175f 100755
+--- a/configure
++++ b/configure
+@@ -19537,9 +19537,9 @@ int main()
+       || fflush ((FILE *)0) < 0
+       || (fpr = fopen ("conftest0", "r")) == (FILE *)0
+       || fread (test_array, 1, 7, fpr) != 7)
+-    exit (1);
++    return 1;
+   else
+-    exit(0); }
++    return 0; }
+ _ACEOF
+ rm -f conftest$ac_exeext
+ if { (ac_try="$ac_link"
+@@ -24022,7 +24022,7 @@ int main()
+   int i;
+   for (i=0; i < B; i++)
+     Array[i] = i - 3;
+-  exit (Array[1] != -2);
++  return Array[1] != -2;
+ }
+ _ACEOF
+ rm -f conftest$ac_exeext
+diff --git a/configure.ac b/configure.ac
+index 8fb7b640864eb4a8..d1f6d7e08c6946d4 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -88,9 +88,9 @@ int main()
+       || fflush ((FILE *)0) < 0
+       || (fpr = fopen ("conftest0", "r")) == (FILE *)0
+       || fread (test_array, 1, 7, fpr) != 7)
+-    exit (1);
++    return 1;
+   else
+-    exit(0); }]])],[AC_MSG_RESULT(yes); AC_DEFINE(HAVE_NULL_FLUSH)],[AC_MSG_RESULT(no)],[AC_MSG_RESULT(no)])
++    return 0; }]])],[AC_MSG_RESULT(yes); AC_DEFINE(HAVE_NULL_FLUSH)],[AC_MSG_RESULT(no)],[AC_MSG_RESULT(no)])
+ 
+ # Checks for header files: ANSI C, POSIX, and nonstandard Unix headers.
+ AC_HEADER_STDC
+@@ -383,7 +383,7 @@ int main()
+   int i;
+   for (i=0; i < B; i++)
+     Array[[i]] = i - 3;
+-  exit (Array[[1]] != -2);
++  return Array[[1]] != -2;
+ }]])],
+ [ac_cv_c_gcc_strength_bug="no"],[ac_cv_c_gcc_strength_bug="yes"],[ac_cv_c_gcc_strength_bug="yes"]) )
+   if test "$ac_cv_c_gcc_strength_bug" = "yes"
diff --git a/plotutils.spec b/plotutils.spec
index 845e6d2..a0400c2 100644
--- a/plotutils.spec
+++ b/plotutils.spec
@@ -1,6 +1,6 @@
 Name:      plotutils
 Version:   2.6
-Release:   29%{?dist}
+Release:   30%{?dist}
 Summary:   GNU vector and raster graphics utilities and libraries
 
 # libxmi is GPLv2+
@@ -11,6 +11,7 @@ Source0:   ftp://ftp.gnu.org/gnu/plotutils/plotutils-%{version}.tar.gz
 Patch0:    plotutils-2.6-png15.patch
 Patch1:    plotutils-aarch64.patch
 Patch2:    plotutils-werror-format-security.patch
+Patch3: plotutils-configure-c99.patch
 
 BuildRequires:   gcc-c++
 BuildRequires:   make
@@ -47,6 +48,9 @@ applications
 %patch0 -p1 -b .png15
 %patch1 -p1 -b .aarch64
 %patch2 -p1 -b .format-security
+%patch3 -p1
+# Avoid attempting autotools rebuild.
+touch -r aclocal.m4 configure*
 
 %build
 %configure --disable-static --enable-libplotter --enable-libxmi --enable-ps-fonts-in-pcl
@@ -93,6 +97,9 @@ rm -f $RPM_BUILD_ROOT%{_infodir}/dir
 
 
 %changelog
+* Wed Jan 18 2023 Florian Weimer <fweimer@redhat.com> - 2.6-30
+- Port configure script to C99
+
 * Fri Jul 22 2022 Fedora Release Engineering <releng@fedoraproject.org> - 2.6-29
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_37_Mass_Rebuild