Fix "could not import: pcre_stack_guard"

kanaka/mal#20 related, seems like some systems have older PCREs, without
pcre_stack_guard. Since it isn't expclitly used, this will *hopefully* prevent
errors from occurring.

src/private/pcre.nim

@@ -10,6 +10,7 @@ when defined(pcreDynlib):
     {.pragma: pcreImport, dynlib: pcreDll.}
   else:
     {.pragma: pcreImport, header: pcreHeader.}
+  {.deadCodeElim: on.}  # Don't error unless unsupported features are used
 else:
   {. passC: "-DHAVE_CONFIG_H", passC: "-I private/pcre_src",
      passL: "-I private/pcre_src" .}