win32: build with -DINITGUID

We always want to use __declspec(selectany) to declare GUIDs, but
manually including <initguid.h> in every file that used GUIDs was
error-prone. Since all <initguid.h> does is define INITGUID and include
<guiddef.h>, we can remove all references to <initguid.h> and just
compile with -DINITGUID to get the same effect.

Also, this partially reverts 622bcb0 by re-adding libuuid.a to the
build, since apparently some GUIDs (such as GUID_NULL) are not declared
in the source file, even when INITGUID is set.

osdep/path-win.c

@@ -17,7 +17,6 @@
 
 #include <windows.h>
 #include <shlobj.h>
-#include <initguid.h>
 #include <knownfolders.h>
 #include <pthread.h>