patch 8.2.3420: _REENTRANT defined more than once

Problem:    _REENTRANT defined more than once.
Solution:   Fix configure script. (Christian Brabandt, closes #8852)
diff --git a/src/auto/configure b/src/auto/configure
index 4f43632..be1440d 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -12978,7 +12978,9 @@
 $as_echo_n "checking for libcanberra... " >&6; }
   ac_save_CFLAGS="$CFLAGS"
   ac_save_LIBS="$LIBS"
-  CFLAGS="$CFLAGS $canberra_cflags"
+  if `echo "$CFLAGS" | grep -v "$canberra_cflags" >/dev/null`; then
+    CFLAGS="$CFLAGS $canberra_cflags"
+  fi
   LIBS="$LIBS $canberra_lib"
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
@@ -14960,7 +14962,7 @@
   fi
 fi
 
-if `echo "$CFLAGS" | grep -v D_XEENTRANT >/dev/null`; then
+if `echo "$CFLAGS" | grep -v D_REENTRANT >/dev/null`; then
   CFLAGS="$CFLAGS -D_REENTRANT"
 fi
 
diff --git a/src/configure.ac b/src/configure.ac
index 4cd6dea..d417b90 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -3756,7 +3756,9 @@
   AC_MSG_CHECKING(for libcanberra)
   ac_save_CFLAGS="$CFLAGS"
   ac_save_LIBS="$LIBS"
-  CFLAGS="$CFLAGS $canberra_cflags"
+  if `echo "$CFLAGS" | grep -v "$canberra_cflags" >/dev/null`; then
+    CFLAGS="$CFLAGS $canberra_cflags"
+  fi
   LIBS="$LIBS $canberra_lib"
   AC_TRY_LINK([
   # include <canberra.h>
diff --git a/src/version.c b/src/version.c
index 664f4ca..6d64ffe 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3420,
+/**/
     3419,
 /**/
     3418,