build: Set libevent minimum version to 2.0.21

Github-Pull: bitcoin#18676
Rebased-From: b68e717

configure.ac

@@ -1237,9 +1237,9 @@ if test x$use_pkgconfig = xyes; then
         BITCOIN_QT_CHECK([PKG_CHECK_MODULES([QR], [libqrencode], [have_qrencode=yes], [have_qrencode=no])])
       fi
       if test x$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests != xnononono; then
-        PKG_CHECK_MODULES([EVENT], [libevent],, [AC_MSG_ERROR(libevent not found.)])
+        PKG_CHECK_MODULES([EVENT], [libevent >= 2.0.21],, [AC_MSG_ERROR(libevent version 2.0.21 or greater not found.)])
         if test x$TARGET_OS != xwindows; then
-          PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads],, [AC_MSG_ERROR(libevent_pthreads not found.)])
+          PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads >= 2.0.21],, [AC_MSG_ERROR(libevent_pthreads version 2.0.21 or greater not found.)])
         fi
       fi
 

doc/dependencies.md

@@ -13,7 +13,7 @@ These are the dependencies currently used by Bitcoin Core. You can find instruct
 | FreeType | [2.7.1](https://download.savannah.gnu.org/releases/freetype) |  | No |  |  |
 | GCC |  | [4.8+](https://gcc.gnu.org/) (C++11 support) |  |  |  |
 | HarfBuzz-NG |  |  |  |  |  |
-| libevent | [2.1.8-stable](https://github.com/libevent/libevent/releases) | 2.0.22 | No |  |  |
+| libevent | [2.1.8-stable](https://github.com/libevent/libevent/releases) | [2.0.21](https://github.com/bitcoin/bitcoin/pull/18676) | No |  |  |
 | libpng |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) |
 | librsvg | |  |  |  |  |
 | MiniUPnPc | [2.0.20180203](http://miniupnp.free.fr/files) |  | No |  |  |