Avoid running init_config() more than once per process

Windows runs `DllMain` multiple times; when new threads are started, for
instance.  We want to run our `DllMain()` implementation only during
process attachment (e.g. when the first thread loads it).

src/libblastrampoline.c

@@ -406,6 +406,9 @@ void * _win32_self_handle;
 BOOL APIENTRY DllMain(HINSTANCE hModule, DWORD code, void *reserved) {
     if (code == DLL_PROCESS_ATTACH) {
         _win32_self_handle = (void *)hModule;
+    } else {
+        // We do not want to run our initialization more than once per process.
+        return TRUE;
     }
 #else
 __attribute__((constructor)) void init(void) {