Drop after FreeBSD 11 EOL around 2021-09-30
https://github.com/freebsd/freebsd-src/commit/92bb8c680916
https://github.com/freebsd/freebsd-src/commit/2b34e8433514

Header <sys/event.h> has symbol "kqueue" : NO
meson.build:62:1: ERROR: Problem encountered: Unsupported system

Compiler stderr:
 In file included from _build/meson-private/tmp0rptdqs6/testfile.c:2:
/usr/include/sys/event.h:75:2: error: unknown type name 'uintptr_t'
        uintptr_t       ident;          /* identifier for this event */
        ^
/usr/include/sys/event.h:77:2: error: unknown type name 'u_short'; did you mean 'short'?
        u_short         flags;          /* action flags for kqueue */
        ^
/usr/include/sys/event.h:78:2: error: unknown type name 'u_int'
        u_int           fflags;         /* filter flag value */
        ^
/usr/include/sys/event.h:79:2: error: unknown type name 'intptr_t'
        intptr_t        data;           /* filter data value */
        ^
In file included from ../src/kqueue.c:20:
/usr/include/sys/event.h:77:2: error: unknown type name 'u_short'; did you mean 'short'?
        u_short         flags;          /* action flags for kqueue */
        ^
/usr/include/sys/event.h:78:2: error: unknown type name 'u_int'
        u_int           fflags;         /* filter flag value */
        ^
../src/kqueue.c:184:20: error: use of undeclared identifier 'NOTE_ABSTIME'
                        NOTE_MSECONDS | NOTE_ABSTIME, deadline, NULL);
                                        ^

--- meson.build.orig	2020-12-31 12:37:18 UTC
+++ meson.build
@@ -50,7 +50,7 @@ sources = [
 ]
 
 have_epoll = cc.has_header_symbol('sys/epoll.h', 'epoll_create')
-have_kqueue = cc.has_header_symbol('sys/event.h', 'kqueue')
+have_kqueue = cc.has_header_symbol('sys/event.h', 'kqueue', prefix: '#include <sys/types.h>')
 
 if have_epoll
 	sources += 'src/epoll.c'
--- src/kqueue.c.orig	2020-12-31 12:37:18 UTC
+++ src/kqueue.c
@@ -17,6 +17,11 @@
 #include "aml.h"
 #include "backend.h"
 
+#if defined(__FreeBSD__) && __FreeBSD__ < 12
+#define NOTE_ABSTIME 0
+typedef unsigned short	u_short;
+typedef unsigned int u_int;
+#endif
 #include <sys/event.h>
 #include <stdlib.h>
 #include <unistd.h>