Use __declspec(selectany) on any windows compiler, not only MSVC

GCC and Clang (in MinGW mode) also do support __declspec(selectany),
and using __declspec(selectany) instead of __attribute__((weak)) on
Windows avoids including the data multiple times in the linked
image. (__attribute__((weak)), on COFF at least, only avoids a conflict
for multiple definitions but doesn't deduplicate the content, like
__declspec(selectany) does.)

This avoids the fact that Clang doesn't fully implement
__attribute__((weak)) on Windows yet.

Author: mstorsjo

amf/public/include/core/Platform.h

@@ -131,7 +131,7 @@
 #endif // WIN32
 
 
-#if defined(_MSC_VER)
+#if defined(_WIN32)
 #define AMF_WEAK __declspec( selectany ) 
 #elif defined (__GNUC__) || defined (__GCC__) || defined(__clang__)//GCC or CLANG
 #define AMF_WEAK __attribute__((weak))