depends: use FORTIFY_SOURCE=3 with libevent

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