diff --git a/opensp-configure-c99.patch b/opensp-configure-c99.patch
new file mode 100644
index 0000000..64e3e4b
--- /dev/null
+++ b/opensp-configure-c99.patch
@@ -0,0 +1,198 @@
+Avoid implicit function declarations because they have been removed
+from C in 1999.  Future compilers are likely to reject them by
+default.
+
+No need to upstream this because the aclocal.m4 file is not checked
+into the repository since:
+
+------------------------------------------------------------------------
+r412 | clasen | 1999-11-06 14:10:34 +0100 (Sat, 06 Nov 1999) | 3 lines
+Changed paths:
+   M /trunk/sp/Makefile.in
+   D /trunk/sp/config/aclocal.m4
+   D /trunk/sp/config/configure.in
+   M /trunk/sp/configure
+   A /trunk/sp/configure.in
+
+Install headers in $(includedir)/OpenSP, move configure.in to toplevel,
+remove generated file aclocal.m4.
+
+diff --git a/aclocal.m4 b/aclocal.m4
+index d7cc45cf7b80ca54..2fde71fb866f12bc 100644
+--- a/aclocal.m4
++++ b/aclocal.m4
+@@ -833,10 +833,6 @@ else
+ #  endif
+ #endif
+ 
+-#ifdef __cplusplus
+-extern "C" void exit (int);
+-#endif
+-
+ void fnord() { int i=42;}
+ int main ()
+ {
+@@ -850,7 +846,7 @@ int main ()
+       /* dlclose (self); */
+     }
+ 
+-    exit (status);
++    return status;
+ }]
+ EOF
+   if AC_TRY_EVAL(ac_link) && test -s conftest${ac_exeext} 2>/dev/null; then
+diff --git a/configure b/configure
+index caa2d31585b112bd..90dce8e854b35393 100755
+--- a/configure
++++ b/configure
+@@ -2917,7 +2917,7 @@ if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
+   echo "$as_me:$LINENO: \$? = $ac_status" >&5
+   (exit $ac_status); }; }; then
+   for ac_declaration in \
+-   '' \
++   '#include <stdlib.h>' \
+    'extern "C" void std::exit (int) throw (); using std::exit;' \
+    'extern "C" void std::exit (int); using std::exit;' \
+    'extern "C" void exit (int) throw ();' \
+@@ -5186,8 +5186,8 @@ main ()
+   for (i = 0; i < 256; i++)
+     if (XOR (islower (i), ISLOWER (i))
+ 	|| toupper (i) != TOUPPER (i))
+-      exit(2);
+-  exit (0);
++      return 2;
++  return 0;
+ }
+ _ACEOF
+ rm -f conftest$ac_exeext
+@@ -9508,10 +9508,6 @@ else
+ #  endif
+ #endif
+ 
+-#ifdef __cplusplus
+-extern "C" void exit (int);
+-#endif
+-
+ void fnord() { int i=42;}
+ int main ()
+ {
+@@ -9525,7 +9521,7 @@ int main ()
+       /* dlclose (self); */
+     }
+ 
+-    exit (status);
++    return status;
+ }
+ EOF
+   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
+@@ -9606,10 +9602,6 @@ else
+ #  endif
+ #endif
+ 
+-#ifdef __cplusplus
+-extern "C" void exit (int);
+-#endif
+-
+ void fnord() { int i=42;}
+ int main ()
+ {
+@@ -9623,7 +9615,7 @@ int main ()
+       /* dlclose (self); */
+     }
+ 
+-    exit (status);
++    return status;
+ }
+ EOF
+   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
+@@ -13321,10 +13313,6 @@ else
+ #  endif
+ #endif
+ 
+-#ifdef __cplusplus
+-extern "C" void exit (int);
+-#endif
+-
+ void fnord() { int i=42;}
+ int main ()
+ {
+@@ -13338,7 +13326,7 @@ int main ()
+       /* dlclose (self); */
+     }
+ 
+-    exit (status);
++    return status;
+ }
+ EOF
+   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
+@@ -13419,10 +13407,6 @@ else
+ #  endif
+ #endif
+ 
+-#ifdef __cplusplus
+-extern "C" void exit (int);
+-#endif
+-
+ void fnord() { int i=42;}
+ int main ()
+ {
+@@ -13436,7 +13420,7 @@ int main ()
+       /* dlclose (self); */
+     }
+ 
+-    exit (status);
++    return status;
+ }
+ EOF
+   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
+@@ -19110,10 +19094,6 @@ else
+ #  endif
+ #endif
+ 
+-#ifdef __cplusplus
+-extern "C" void exit (int);
+-#endif
+-
+ void fnord() { int i=42;}
+ int main ()
+ {
+@@ -19127,7 +19107,7 @@ int main ()
+       /* dlclose (self); */
+     }
+ 
+-    exit (status);
++    return status;
+ }
+ EOF
+   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
+@@ -19208,10 +19188,6 @@ else
+ #  endif
+ #endif
+ 
+-#ifdef __cplusplus
+-extern "C" void exit (int);
+-#endif
+-
+ void fnord() { int i=42;}
+ int main ()
+ {
+@@ -19225,7 +19201,7 @@ int main ()
+       /* dlclose (self); */
+     }
+ 
+-    exit (status);
++    return status;
+ }
+ EOF
+   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
+@@ -27450,8 +27426,8 @@ 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
diff --git a/opensp.spec b/opensp.spec
index 0b5d24e..734909b 100644
--- a/opensp.spec
+++ b/opensp.spec
@@ -1,7 +1,7 @@
 Summary: SGML and XML parser
 Name: opensp
 Version: 1.5.2
-Release: 40%{?dist}
+Release: 41%{?dist}
 Requires: sgml-common >= 0.5
 Provides: bundled(gettext) = 0.14.5
 URL: http://openjade.sourceforge.net/
@@ -10,6 +10,7 @@ Patch0: opensp-multilib.patch
 Patch1: opensp-nodeids.patch
 Patch2: opensp-sigsegv.patch
 Patch3: opensp-manpage.patch
+Patch4: opensp-configure-c99.patch
 License: MIT
 
 BuildRequires: make
@@ -39,6 +40,7 @@ Header files and libtool library for developing applications that use OpenSP.
 %patch1 -p1 -b .nodeids
 %patch2 -p1 -b .sigsegv
 %patch3 -p1 -b .manpage
+%patch4 -p1 -b .configure-c99
 # convert files to UTF-8
 iconv -f latin1 -t utf8 ChangeLog -o ChangeLog.tmp
 mv -f ChangeLog.tmp ChangeLog
@@ -117,6 +119,9 @@ make check || : # TODO: failures as of 1.5.2 :(
 
 
 %changelog
+* Tue Dec  6 2022 Florian Weimer <fweimer@redhat.com> - 1.5.2-41
+- Port configure script to C99
+
 * Fri Jul 22 2022 Fedora Release Engineering <releng@fedoraproject.org> - 1.5.2-40
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_37_Mass_Rebuild