diff --git a/libprelude-configure-c99.patch b/libprelude-configure-c99.patch
new file mode 100644
index 0000000..ec136b0
--- /dev/null
+++ b/libprelude-configure-c99.patch
@@ -0,0 +1,82 @@
+Add missing include files to avoid implicit function declarations.
+
+diff --git a/configure b/configure
+index 3dd46a982bc30af1..0fd5bd685c32f3cd 100755
+--- a/configure
++++ b/configure
+@@ -22292,6 +22292,7 @@ else
+         cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ #include <stdarg.h>
++	#include <stdlib.h>
+         void f (int i, ...) {
+         va_list args1, args2;
+         va_start (args1, i);
+@@ -22325,6 +22326,7 @@ else
+         cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ #include <stdarg.h>
++	#include <stdlib.h>
+         void f (int i, ...) {
+         va_list args1, args2;
+         va_start (args1, i);
+@@ -22377,6 +22379,7 @@ else
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ #include <stdarg.h>
++	#include <stdlib.h>
+         void f (int i, ...) {
+         va_list args1, args2;
+         va_start (args1, i);
+@@ -48608,6 +48611,9 @@ else
+ #if HAVE_LANGINFO_CODESET
+ # include <langinfo.h>
+ #endif
++#if HAVE_WCTYPE_H
++# include <wctype.h>
++#endif
+ #include <stdlib.h>
+ #include <string.h>
+ struct tm t;
+diff --git a/configure.ac b/configure.ac
+index eb506e7d2af0afee..e268386180b5e524 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -344,6 +344,7 @@ dnl all results in config.log for bug reports.
+ 
+ AC_CACHE_CHECK([for an implementation of va_copy()],prelude_cv_va_copy,[
+         AC_LINK_IFELSE([AC_LANG_SOURCE([#include <stdarg.h>
++	#include <stdlib.h>
+         void f (int i, ...) {
+         va_list args1, args2;
+         va_start (args1, i);
+@@ -361,6 +362,7 @@ AC_CACHE_CHECK([for an implementation of va_copy()],prelude_cv_va_copy,[
+ ])
+ AC_CACHE_CHECK([for an implementation of __va_copy()],prelude_cv___va_copy,[
+         AC_LINK_IFELSE([AC_LANG_SOURCE([#include <stdarg.h>
++	#include <stdlib.h>
+         void f (int i, ...) {
+         va_list args1, args2;
+         va_start (args1, i);
+@@ -390,6 +392,7 @@ fi
+ 
+ AC_CACHE_CHECK([whether va_lists can be copied by value],prelude_cv_va_val_copy,[
+         AC_TRY_RUN([#include <stdarg.h>
++	#include <stdlib.h>
+         void f (int i, ...) {
+         va_list args1, args2;
+         va_start (args1, i);
+diff --git a/libmissing/m4/locale-tr.m4 b/libmissing/m4/locale-tr.m4
+index 2f2cf47892d505a5..c2683e53b8574f6d 100644
+--- a/libmissing/m4/locale-tr.m4
++++ b/libmissing/m4/locale-tr.m4
+@@ -19,6 +19,9 @@ changequote(,)dnl
+ #if HAVE_LANGINFO_CODESET
+ # include <langinfo.h>
+ #endif
++#if HAVE_WCTYPE_H
++# include <wctype.h>
++#endif
+ #include <stdlib.h>
+ #include <string.h>
+ struct tm t;
diff --git a/libprelude.spec b/libprelude.spec
index 9a867a3..fff1204 100644
--- a/libprelude.spec
+++ b/libprelude.spec
@@ -7,7 +7,7 @@
 
 Name:           libprelude
 Version:        5.2.0
-Release:        14%{?dist}
+Release:        15%{?dist}
 Summary:        Secure Connections between all Sensors and the Prelude Manager
 License:        LGPL-2.1+
 URL:            https://www.prelude-siem.org/
@@ -39,6 +39,7 @@ Patch11:        libprelude-5.2.0-ruby27.patch
 Patch12:        libprelude-5.2.0-clean_libprelude-config.patch
 # #1973946
 Patch13:        libprelude-5.2.0-fix-PyIOBase_Type.patch
+Patch14:        libprelude-configure-c99.patch
 BuildRequires: make
 BuildRequires:  gcc
 BuildRequires:  gcc-c++
@@ -241,6 +242,9 @@ patch -d %{buildroot}%{_includedir}/libprelude/ -p0 < %SOURCE1
 %doc AUTHORS ChangeLog README NEWS
 
 %changelog
+* Fri Nov 25 2022 Florian Weimer <fweimer@redhat.com> - 5.2.0-15
+- Port configure script to C99 (#2148366)
+
 * Thu Jul 21 2022 Fedora Release Engineering <releng@fedoraproject.org> - 5.2.0-14
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_37_Mass_Rebuild