[Varnish] #1230: Refine header check in bin/varnishd/mgt/mgt.h, mgt_main.c for uClibc

[Varnish]

#1230: Refine header check in bin/varnishd/mgt/mgt.h, mgt_main.c  for uClibc
----------------------+--------------------
 Reporter:  basile@…  |       Type:  defect
   Status:  new       |   Priority:  normal
Milestone:            |  Component:  build
  Version:  trunk     |   Severity:  normal
 Keywords:            |
----------------------+--------------------
 Because of the difference in how uClibc and glibc stack their header
 files, stdio.h indirectly brings in PTHREAD_CANCELED from pthread.h on a
 uClibc system, whereas it does not on a glibc system.  This happens in
 mgt.h and mgt_main.c, and causes a (pre-)compile time failure.  This patch
 refines the check in those files to take this fact into consideration.

 See the downstream bug for more details: https://bugs.gentoo.org/444294

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1230>
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