Fix DECREF name clash under MinGW

MinGW's winnt.h also defines DECREF.

Also fix unused variable warnings in the Windows TLS code.

Fixes CLOWNFISH-101.
diff --git a/runtime/c/src/tls.c b/runtime/c/src/tls.c
index b550f13..e805dbc 100644
--- a/runtime/c/src/tls.c
+++ b/runtime/c/src/tls.c
@@ -74,12 +74,15 @@
 
 BOOL WINAPI
 DllMain(HINSTANCE dll, DWORD reason, LPVOID reserved) {
+    UNUSED_VAR(dll);
+    UNUSED_VAR(reserved);
+
     if (reason == DLL_THREAD_DETACH) {
         ErrContext *context
             = (ErrContext*)TlsGetValue(err_context_tls_index);
 
         if (context) {
-            DECREF(context->current_error);
+            CFISH_DECREF(context->current_error);
             FREEMEM(context);
         }
     }