Merge bitcoin/bitcoin#27118: depends: harden libevent

ff4a73a depends: use FORTIFY_SOURCE=3 with libevent (fanquake)

Pull request description:

  Use `FORTIFY_SOURCE=3` when building libevent in depends. I've upstreamed a change to switch libevent from using =2 to =3 as well: libevent/libevent#1418.

  Solves half of #27038, by giving us some fortified funcs in `bitcoin-cli`.

ACKs for top commit:
  TheCharlatan:
    ACK ff4a73a

Tree-SHA512: eaf692ec92b288f0cb524c011fc81529f58efa4c43d418a7b3ae7108eba2bccba708a81a28ac6d063267be80ca615637c6e3fccc02497d7367af2eaae0e8d812

Author: fanquake

depends/packages/libevent.mk

@@ -16,6 +16,7 @@ define $(package)_set_vars
   $(package)_config_opts_netbsd=--with-pic
   $(package)_config_opts_openbsd=--with-pic
   $(package)_config_opts_android=--with-pic
+  $(package)_cppflags+=-D_FORTIFY_SOURCE=3
   $(package)_cppflags_mingw32=-D_WIN32_WINNT=0x0601
 endef