diff --git a/Makefile.in b/Makefile.in
index 91d6281b53..4cd7651056 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -68,6 +68,7 @@ HOST_SUFFIX = @HOST_SUFFIX@
 DYLIB_RPATH_INSTALL = @DYLIB_RPATH_INSTALL@
 DYLIB_RPATH_POSTLINK = @DYLIB_RPATH_POSTLINK@
 wx_top_builddir = @wx_top_builddir@
+wxCFLAGS_C99 = @wxCFLAGS_C99@
 
 ### Variables: ###
 
@@ -211,7 +212,7 @@ WXTIFF_OBJECTS =  \
 	wxtiff_tif_zip.o \
 	wxtiff_tif_zstd.o
 WXEXPAT_CFLAGS = -DNDEBUG -I./src/expat/expat -DHAVE_EXPAT_CONFIG_H \
-	$(____SHARED) $(CPPFLAGS) $(CFLAGS)
+	$(____SHARED) $(wxCFLAGS_C99) $(CPPFLAGS) $(CFLAGS)
 WXEXPAT_OBJECTS =  \
 	wxexpat_xmlparse.o \
 	wxexpat_xmlrole.o \
diff --git a/build/bakefiles/expat.bkl b/build/bakefiles/expat.bkl
index 5cef4ed12e..0f01e4da55 100644
--- a/build/bakefiles/expat.bkl
+++ b/build/bakefiles/expat.bkl
@@ -4,6 +4,7 @@
 
     <if cond="FORMAT=='autoconf'">
         <option name="wxUSE_EXPAT"/>
+        <option name="wxCFLAGS_C99"/>
         <set var="LIB_EXPAT">
             <if cond="wxUSE_EXPAT=='builtin'">
                 wxexpat$(WXDEBUGFLAG)$(WX_LIB_FLAVOUR)-$(WX_RELEASE)$(HOST_SUFFIX)
@@ -31,6 +32,7 @@
             HAVE_EXPAT_CONFIG_H
         </define>
         <cflags-borland>-w-8004 -w-8008 -w-8012 -w-8057 -w-8066</cflags-borland>
+        <cflags cond="FORMAT=='autoconf'">$(wxCFLAGS_C99)</cflags>
         <sources>
             src/expat/expat/lib/xmlparse.c
             src/expat/expat/lib/xmlrole.c
diff --git a/configure b/configure
index e6ec44da26..e289050304 100755
--- a/configure
+++ b/configure
@@ -976,6 +976,7 @@ DIRECTFB_CFLAGS
 GTK_CONFIG
 GTK_LIBS
 GTK_CFLAGS
+wxCFLAGS_C99
 subdirs
 LIBTIFF_LIBS
 LIBTIFF_CFLAGS
@@ -23785,6 +23786,189 @@ $as_echo "$as_me: WARNING: system expat library not found, will use built-in ins
         fi
     fi
     if test "$wxUSE_EXPAT" = "builtin" ; then
+                        save_CC="$CC"
+           { $as_echo "$as_me:${as_lineno-$LINENO}: checking for $CC option to accept ISO C99" >&5
+$as_echo_n "checking for $CC option to accept ISO C99... " >&6; }
+if ${ac_cv_prog_cc_c99+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_cv_prog_cc_c99=no
+ac_save_CC=$CC
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <wchar.h>
+#include <stdio.h>
+
+// Check varargs macros.  These examples are taken from C99 6.10.3.5.
+#define debug(...) fprintf (stderr, __VA_ARGS__)
+#define showlist(...) puts (#__VA_ARGS__)
+#define report(test,...) ((test) ? puts (#test) : printf (__VA_ARGS__))
+static void
+test_varargs_macros (void)
+{
+  int x = 1234;
+  int y = 5678;
+  debug ("Flag");
+  debug ("X = %d\n", x);
+  showlist (The first, second, and third items.);
+  report (x>y, "x is %d but y is %d", x, y);
+}
+
+// Check long long types.
+#define BIG64 18446744073709551615ull
+#define BIG32 4294967295ul
+#define BIG_OK (BIG64 / BIG32 == 4294967297ull && BIG64 % BIG32 == 0)
+#if !BIG_OK
+  your preprocessor is broken;
+#endif
+#if BIG_OK
+#else
+  your preprocessor is broken;
+#endif
+static long long int bignum = -9223372036854775807LL;
+static unsigned long long int ubignum = BIG64;
+
+struct incomplete_array
+{
+  int datasize;
+  double data[];
+};
+
+struct named_init {
+  int number;
+  const wchar_t *name;
+  double average;
+};
+
+typedef const char *ccp;
+
+static inline int
+test_restrict (ccp restrict text)
+{
+  // See if C++-style comments work.
+  // Iterate through items via the restricted pointer.
+  // Also check for declarations in for loops.
+  for (unsigned int i = 0; *(text+i) != '\0'; ++i)
+    continue;
+  return 0;
+}
+
+// Check varargs and va_copy.
+static void
+test_varargs (const char *format, ...)
+{
+  va_list args;
+  va_start (args, format);
+  va_list args_copy;
+  va_copy (args_copy, args);
+
+  const char *str;
+  int number;
+  float fnumber;
+
+  while (*format)
+    {
+      switch (*format++)
+	{
+	case 's': // string
+	  str = va_arg (args_copy, const char *);
+	  break;
+	case 'd': // int
+	  number = va_arg (args_copy, int);
+	  break;
+	case 'f': // float
+	  fnumber = va_arg (args_copy, double);
+	  break;
+	default:
+	  break;
+	}
+    }
+  va_end (args_copy);
+  va_end (args);
+}
+
+int
+main ()
+{
+
+  // Check bool.
+  _Bool success = false;
+
+  // Check restrict.
+  if (test_restrict ("String literal") == 0)
+    success = true;
+  char *restrict newvar = "Another string";
+
+  // Check varargs.
+  test_varargs ("s, d' f .", "string", 65, 34.234);
+  test_varargs_macros ();
+
+  // Check flexible array members.
+  struct incomplete_array *ia =
+    malloc (sizeof (struct incomplete_array) + (sizeof (double) * 10));
+  ia->datasize = 10;
+  for (int i = 0; i < ia->datasize; ++i)
+    ia->data[i] = i * 1.234;
+
+  // Check named initializers.
+  struct named_init ni = {
+    .number = 34,
+    .name = L"Test wide string",
+    .average = 543.34343,
+  };
+
+  ni.number = 58;
+
+  int dynamic_array[ni.number];
+  dynamic_array[ni.number - 1] = 543;
+
+  // work around unused variable warnings
+  return (!success || bignum == 0LL || ubignum == 0uLL || newvar[0] == 'x'
+	  || dynamic_array[ni.number - 1] != 543);
+
+  ;
+  return 0;
+}
+_ACEOF
+for ac_arg in '' -std=gnu99 -std=c99 -c99 -AC99 -D_STDC_C99= -qlanglvl=extc99
+do
+  CC="$ac_save_CC $ac_arg"
+  if ac_fn_c_try_compile "$LINENO"; then :
+  ac_cv_prog_cc_c99=$ac_arg
+fi
+rm -f core conftest.err conftest.$ac_objext
+  test "x$ac_cv_prog_cc_c99" != "xno" && break
+done
+rm -f conftest.$ac_ext
+CC=$ac_save_CC
+
+fi
+# AC_CACHE_VAL
+case "x$ac_cv_prog_cc_c99" in
+  x)
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: none needed" >&5
+$as_echo "none needed" >&6; } ;;
+  xno)
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: unsupported" >&5
+$as_echo "unsupported" >&6; } ;;
+  *)
+    CC="$CC $ac_cv_prog_cc_c99"
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_prog_cc_c99" >&5
+$as_echo "$ac_cv_prog_cc_c99" >&6; } ;;
+esac
+if test "x$ac_cv_prog_cc_c99" != xno; then :
+
+fi
+
+
+        CC="$save_CC"
+
+                        wxCFLAGS_C99=$ac_cv_prog_cc_c99
+
+
                 subdirs="$subdirs src/expat/expat"
 
     fi
diff --git a/configure.in b/configure.in
index 0d498f3ea9..fc8b1de49b 100644
--- a/configure.in
+++ b/configure.in
@@ -2721,6 +2721,17 @@ if test "$wxUSE_EXPAT" != "no"; then
         fi
     fi
     if test "$wxUSE_EXPAT" = "builtin" ; then
+        dnl Expat requires C99 compiler, so define wxCFLAGS_C99 variable which
+        dnl is used when compiling it in our makefiles.
+        save_CC="$CC"
+        AC_PROG_CC_C99
+        CC="$save_CC"
+
+        dnl Note that we need just the flags, not the full CC value including
+        dnl the compiler too.
+        wxCFLAGS_C99=$ac_cv_prog_cc_c99
+        AC_SUBST(wxCFLAGS_C99)
+
         dnl Expat needs this:
         AC_CONFIG_SUBDIRS([src/expat/expat])
     fi