Revert "Stop checking whether environ is declared."

This reverts commit 954eeda619a59dae76144ad69967f0ed7341b564.

Apparently, OpenBSD is what expects you to declare environ
yourself.  So 19142 is a wontfix.

changes/19142

@@ -1,3 +0,0 @@
-  o Removed features:
-    - We no longer attempt to build on systems where 'environ' is not
-      declared in the C headers. Closes ticket 19142.

configure.ac

@@ -1535,6 +1535,17 @@ int main(int c, char **v) { puts(__FUNCTION__); }])],
   tor_cv_have_FUNCTION_macro=yes,
   tor_cv_have_FUNCTION_macro=no))
 
+AC_CACHE_CHECK([whether we have extern char **environ already declared],
+  tor_cv_have_environ_declared,
+  AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+#include <stdlib.h>
+int main(int c, char **v) { char **t = environ; }])],
+  tor_cv_have_environ_declared=yes,
+  tor_cv_have_environ_declared=no))
+
 if test "$tor_cv_have_func_macro" = "yes"; then
   AC_DEFINE(HAVE_MACRO__func__, 1, [Defined if the compiler supports __func__])
 fi
@@ -1548,6 +1559,11 @@ if test "$tor_cv_have_FUNCTION_macro" = "yes"; then
            [Defined if the compiler supports __FUNCTION__])
 fi
 
+if test "$tor_cv_have_environ_declared" = "yes"; then
+  AC_DEFINE(HAVE_EXTERN_ENVIRON_DECLARED, 1,
+           [Defined if we have extern char **environ already declared])
+fi
+
 # $prefix stores the value of the --prefix command line option, or
 # NONE if the option wasn't set.  In the case that it wasn't set, make
 # it be the default, so that we can use it to expand directories now.

src/common/compat.c

@@ -2390,6 +2390,15 @@ make_path_absolute(char *fname)
 #endif
 }
 
+#ifndef HAVE__NSGETENVIRON
+#ifndef HAVE_EXTERN_ENVIRON_DECLARED
+/* Some platforms declare environ under some circumstances, others don't. */
+#ifndef RUNNING_DOXYGEN
+extern char **environ;
+#endif
+#endif
+#endif
+
 /** Return the current environment. This is a portable replacement for
  * 'environ'. */
 char **