os.execShellCmd: fixes #10231 (#10232)

Darwin has long deprecated the wait union, but their macros still assume
it unless you define _POSIX_C_SOURCE. This trips up C++ compilers.

This commit duplicates the behavior of WEXITSTATUS when _POSIX_C_SOURCE
is defined.

tests/stdlib/t10231.nim

@@ -0,0 +1,13 @@
+discard """
+  target: cpp
+  action: run
+  exitcode: 0
+"""
+
+import os
+
+if paramCount() == 0:
+  # main process
+  doAssert execShellCmd(getAppFilename().quoteShell & " test") == 1
+else:
+  quit 1