Do load SHDefExtractIcon() during run-time for old MinGW

The problem with MinGW 5.3 is not just due to the missing function
declaration in the header, but also due to the function missing from
libshell32.a import library, so we do need to load it during run-time,
contrary to what the comment in the previous commit said.

This should finally fix MinGW build.

src/msw/artmsw.cpp

@@ -31,30 +31,42 @@
     #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
 {
 
+#ifdef SHDefExtractIcon
+    #define MSW_SHDefExtractIcon SHDefExtractIcon
+#else // !defined(SHDefExtractIcon)
+
+// MinGW doesn't provide SHDefExtractIcon() up to at least the 5.3 version, so
+// define it ourselves.
+HRESULT
+MSW_SHDefExtractIcon(LPCTSTR pszIconFile, int iIndex, UINT uFlags,
+                     HICON *phiconLarge, HICON *phiconSmall, UINT nIconSize)
+{
+    typedef HRESULT
+    (WINAPI *SHDefExtractIcon_t)(LPCTSTR, int, UINT, HICON*, HICON*, UINT);
+
+    static SHDefExtractIcon_t s_SHDefExtractIcon = NULL;
+    if ( !s_SHDefExtractIcon )
+    {
+        wxDynamicLibrary shell32(wxT("shell32.dll"));
+        wxDL_INIT_FUNC_AW(s_, SHDefExtractIcon, shell32);
+
+        if ( !s_SHDefExtractIcon )
+            return E_FAIL;
+
+        // Prevent the DLL from being unloaded while we use its function.
+        // Normally it's not a problem as shell32.dll is always loaded anyhow.
+        shell32.Detach();
+    }
+
+    return (*s_SHDefExtractIcon)(pszIconFile, iIndex, uFlags,
+                                 phiconLarge, phiconSmall, nIconSize);
+}
+
+#endif // !defined(SHDefExtractIcon)
+
 #ifdef wxHAS_SHGetStockIconInfo
 
 SHSTOCKICONID MSWGetStockIconIdForArtProviderId(const wxArtID& art_id)
@@ -111,7 +123,7 @@ wxBitmap
 MSWGetBitmapFromIconLocation(const TCHAR* path, int index, const wxSize& size)
 {
     HICON hIcon = NULL;
-    if ( SHDefExtractIcon(path, index, 0, &hIcon, NULL, size.x) != S_OK )
+    if ( MSW_SHDefExtractIcon(path, index, 0, &hIcon, NULL, size.x) != S_OK )
         return wxNullBitmap;
 
     // Note that using "size.x" twice here is not a typo: normally size.y is