idris_support: fix environ for macOS (#3324)

* idris_support: fix environ for macOS

* Add test that touches the new implementation of environ

---------

Co-authored-by: Sergey Fedorov <barracuda@macos-powerpc.org>
Co-authored-by: Mathew Polzin <matt.polzin@gmail.com>

support/c/idris_support.c

@@ -23,6 +23,9 @@ char **_argv;
 extern char **_environ;
 #include "windows/win_utils.h"
 #define environ _environ
+#elif defined(__APPLE__)
+#include <crt_externs.h>
+#define environ (*_NSGetEnviron())
 #else
 extern char **environ;
 #endif

tests/base/system_get_environment/Test.idr

@@ -0,0 +1,8 @@
+import System
+
+main : IO ()
+main = do
+  env <- getEnvironment
+  -- it better be non-empty because we set a variable before starting this test
+  printLn $ null env
+

tests/base/system_get_environment/expected

@@ -0,0 +1 @@
+False

tests/base/system_get_environment/run

@@ -0,0 +1,4 @@
+. ../../testutils.sh
+
+idris2 --cg chez -o test Test.idr
+HELLO=hi ./build/exec/test