Thread-local support for more Windows toolchains.

Use of thread-local storage on Windows (introduced recently in
69e8d471d1) could cause a -Wattributes warning in mingw-w64 builds,
since that toolchain doesn't understand __declspec(thread).

Define a portability macro THREADLOCAL, which should resolve to
something appropriate for at least:
 - MSVC, which understands the Microsoft syntax __declspec(thread);
 - GCC (e.g., mingw-w64) which understands the GNU syntax __thread
   (GCC only implements __declspec() to the extent of understanding the
   arguments 'dllexport' and 'dllimport');
 - Clang, which supports both syntaxes.

(It's possible there's some more obscure Windows toolchain which will
now hit the #error as a result of this change.)

I haven't attempted to try to detect and use the C11 syntax
'thread_local'. And this is all still Windows-only, since that's all we
need for now and it avoids opening the can of worms that is TLS on other
platforms.

(I considered delegating all this to cmake, but as well as being fiddly,
it seems even the latest versions of cmake don't know about thread-local
storage for C, as opposed to C++ (cxx_thread_local).)

windows/platform.h

@@ -35,6 +35,14 @@
 #define BUILDINFO_PLATFORM "Windows"
 #endif
 
+#if defined __GNUC__ || defined __clang__
+#define THREADLOCAL __thread
+#elif defined _MSC_VER
+#define THREADLOCAL __declspec(thread)
+#else
+#error Do not know how to declare thread-local storage with this toolchain
+#endif
+
 /* Randomly-chosen dwData value identifying a WM_COPYDATA message as
  * being a Pageant transaction */
 #define AGENT_COPYDATA_ID 0x804e50ba

windows/utils/shinydialogbox.c

@@ -31,7 +31,7 @@ struct ShinyDialogBoxState {
  * threads at all, let alone concurrently, but just in case, declaring
  * sdb_tempstate as thread-local will protect against that possibility.
  */
-static __declspec(thread) struct ShinyDialogBoxState *sdb_tempstate;
+static THREADLOCAL struct ShinyDialogBoxState *sdb_tempstate;
 
 static inline struct ShinyDialogBoxState *ShinyDialogGetState(HWND hwnd)
 {