[tor-bugs] #21023 [Core Tor/Tor]: Replace custom checks with Autoconf macros

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#21023: Replace custom checks with Autoconf macros
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     Reporter:  cypherpunks   |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: unspecified
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 Removing environ in #19142 didn't work on BSDs but the check in
 `configure.ac` can be simplified. Autoconf provides the `AC_CHECK_DECL`
 macro for these kinds of checks and turns the current check into a
 oneliner.

 The `malloc(0)` check can also be simplified by replacing it with the
 `AC_FUNC_MALLOC` macro.

 These are only two candidates I've found so far, there are probably more
 hence this ticket.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21023>
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