Declare SHDefExtractIcon() if it's missing from (MinGW) headers

At least the MinGW 5.3 used for AppVeyor builds doesn't declare this
function, so provide its declaration ourselves to fix MinGW build after
the recent changes.

src/msw/artmsw.cpp

@@ -31,6 +31,27 @@
     #define wxHAS_SHGetStockIconInfo
 #endif
 
+// MinGW headers don't always define this symbol (up to at least 5.3 version),
+// so do it ourselves.
+//
+// Note that at least there is no need for run-time loading here, as it's
+// available since XP.
+#ifndef SHDefExtractIcon
+
+#ifdef UNICODE
+extern "C" HRESULT wxSTDCALL
+SHDefExtractIconW(LPCWSTR pszIconFile, int iIndex, UINT uFlags,
+                  HICON *phiconLarge, HICON *phiconSmall, UINT nIconSize);
+#define SHDefExtractIcon SHDefExtractIconW
+#else // !UNICODE
+extern "C" HRESULT wxSTDCALL
+SHDefExtractIconA(LPCSTR pszIconFile, int iIndex, UINT uFlags,
+                  HICON *phiconLarge, HICON *phiconSmall, UINT nIconSize);
+#define SHDefExtractIcon SHDefExtractIconA
+#endif // UNICODE/!UNICODE
+
+#endif // !defined(SHDefExtractIcon)
+
 namespace
 {