[tbb-bugs] #26249 [Applications/Tor Browser]: Update libevent to prevent broken compilation when targetting macOS 10.7

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#26249: Update libevent to prevent broken compilation when targetting macOS 10.7
-------------------------------------+-------------------------------------
     Reporter:  gk                   |      Owner:  tbb-team
         Type:  defect               |     Status:  new
     Priority:  Very High            |  Milestone:
    Component:  Applications/Tor     |    Version:
  Browser                            |   Keywords:  tbb-rbm, ff60-esr,
     Severity:  Normal               |  TorBrowserTeam201805,
                                     |  GeorgKoppen201805
Actual Points:                       |  Parent ID:  #24632
       Points:                       |   Reviewer:
      Sponsor:                       |
-------------------------------------+-------------------------------------
 Switching our toolchain to target macOS 10.7 using the 10.11 SDK breaks
 compilation of libevent:
 {{{
 evutil.c:428:16: error: variable has incomplete type 'struct _timeb'
         struct _timeb tb;
                       ^
 evutil.c:428:9: note: forward declaration of 'struct _timeb'
         struct _timeb tb;
                ^
 evutil.c:441:2: warning: implicit declaration of function '_ftime' is
 invalid in C99 [-Wimplicit-function-declaration]
         _ftime(&tb);
         ^
 In file included from evutil_rand.c:125:
 ././arc4random.c:465:1: error: static declaration of
 'arc4random_addrandom' follows non-static declaration
 arc4random_addrandom(const unsigned char *dat, int datlen)
 ^
 /var/tmp/dist/macosx-toolchain/SDK//usr/include/stdlib.h:244:7: note:
 previous declaration is here
 void     arc4random_addrandom(unsigned char * /*dat*/, int /*datlen*/);
          ^
 In file included from evutil_rand.c:125:
 ././arc4random.c:497:1: error: static declaration of 'arc4random_buf'
 follows non-static declaration
 arc4random_buf(void *_buf, size_t n)
 ^
 /var/tmp/dist/macosx-toolchain/SDK//usr/include/stdlib.h:245:7: note:
 previous declaration is here
 void     arc4random_buf(void * /*buf*/, size_t /*nbytes*/)
 __OSX_AVAILABLE_STARTING(__MAC_10_7, __IPHONE_4_3);
          ^
 2 errors generated.
 }}}
 The error in `evutils.c` is fixed in the latest stable version, 2.1.8,
 which we might want to use from now on. However the `arc4random.c` ones
 persist, even on `master`.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26249>
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