[PATCH 0/1] MR8494: include: Fix __uuidof(IUnknown) and IID_IUnknown in MinGW C++.

[Alfred Agrell (＠Alcaro)]

Before:
```c++
#define INITGUID
#include <unknwn.h>
const GUID& a() { return __uuidof(IUnknown); }
const GUID& b() { return IID_IUnknown; }
```
```
$ clang -xc++ -target i386-pc-windows-gnu -nostdinc -I include -I include/msvcrt a.cpp -masm=intel -nostdlib
a.cpp:3:26: warning: instantiation of variable '__wine_uuidof<IUnknown>::uuid' required here, but no definition is available [-Wundefined-var-template]
[...]
1 warning generated.
/usr/bin/ld: /tmp/a-7e5d2f.o:a.cpp:(.rdata$.refptr._IID_IUnknown+0x0): undefined reference to `IID_IUnknown'
/usr/bin/ld: /tmp/a-7e5d2f.o:a.cpp:(.rdata$.refptr.__ZN13__wine_uuidofI8IUnknownE4uuidE+0x0): undefined reference to `__wine_uuidof<IUnknown>::uuid'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```
After: Creates an a.exe with no warnings. (With a() as entry point; figuring out how to give it a real entry point is offtopic to this MR.)

[PATCH 1/1] include: Enable __uuidof(IUnknown)

[Alfred Agrell]

From: Alfred Agrell

---
 include/unknwn.idl | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/unknwn.idl b/include/unknwn.idl
index 0798ccf79ab..7c4a8f02015 100644
--- a/include/unknwn.idl
+++ b/include/unknwn.idl
@@ -60,6 +60,9 @@ cpp_quote("        }")
 cpp_quote("        END_INTERFACE")
 cpp_quote("    };")
 cpp_quote("}")
+cpp_quote("#ifdef __CRT_UUID_DECL")
+cpp_quote("__CRT_UUID_DECL(IUnknown, 0x00000000, 0x0000, 0x0000, 0xc0,0x00, 0x00,0x00,0x00,0x00,0x00,0x46)")
+cpp_quote("#endif")
 cpp_quote("#else")
   HRESULT QueryInterface(
     [in] REFIID riid,