Replaced __declspec(thread) with TlsAlloc.

Variables created within a DLL with __declspec(thread) may not get a TLS
slot on Windows XP, leading to segfaults on use.  Moving to TlsAlloc
works around this.

src/wgl_platform.h

@@ -76,7 +76,8 @@ typedef struct _GLFWcontextWGL
 //------------------------------------------------------------------------
 typedef struct _GLFWlibraryWGL
 {
-    int dummy;
+    GLboolean   hasTLS;
+    DWORD       tls;
 
 } _GLFWlibraryWGL;