[tor-bugs] #19139 [Core Tor/Tor]: Let Autoconf handle enabling C and POSIX extensions

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#19139: Let Autoconf handle enabling C and POSIX extensions
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     Reporter:  cypherpunks   |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Minor         |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 The comment on
 [https://gitweb.torproject.org/tor.git/tree/src/common/compat.c?id=9c7edb0f3edd2dec30f10e195f7bcc12a4005f30#n18
 src/common/compat.c:18] suggests using `AC_USE_SYSTEM_EXTENSIONS`. The
 attached patch does just that.

 Defining the preprocessor macros globally ensures the extensions are
 always turned on. Running GCC in pedantic mode tells me that there are
 instances where the code assumes the extensions are enabled while missing
 the proper preprocessor macros. FWICT it currently ''just works'' because
 GCC and its headers turn their own extensions on by default.

 The comment suggestion about maybe removing the `environ` configure check
 has not been researched.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19139>
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