configure: mingw32 targets: test for WIC support

Replace usage of _WIN32 in examples as this does not guarantee the
presence of wincodec.h.
mingw-w64 notably includes wincodec.h, though other releases do not.

Under cygwin the following can be used for a WIC enabled binary:
./configure --target=i686-pc-mingw32 CC=i686-w64-mingw32-gcc.exe

Change-Id: Ica6a714c3356a8eaf88486a1c3f5aa6adde394c0

configure.ac

@@ -1,4 +1,5 @@
 AC_INIT([webpdecode], [0.1])
+AC_CANONICAL_TARGET
 AM_INIT_AUTOMAKE([-Wall foreign subdir-objects])
 AC_PROG_LIBTOOL
 AM_PROG_CC_C_O
@@ -74,6 +75,49 @@ AC_CHECK_HEADER(jpeglib.h,
 AC_SUBST(JPEG_LIBS)
 AC_SUBST(JPEG_INCLUDES)
 
+dnl === check for WIC support ===
+
+if test "$target_os" = "mingw32"; then
+  AC_CHECK_HEADERS([wincodec.h shlwapi.h windows.h])
+  if test "$ac_cv_header_wincodec_h" = "yes"; then
+    AC_MSG_CHECKING(for Windows Imaging Component support)
+    saved_LIBS=$LIBS
+    LIBS="-lshlwapi -lole32 $LIBS"
+    # match include structure from [cd]webp.c
+    wic_headers="
+      #define INITGUID
+      #define CINTERFACE
+      #define COBJMACROS
+      #define _WIN32_IE 0x500
+
+      #include <shlwapi.h>
+      #include <windows.h>
+      #include <wincodec.h>
+      "
+    # test for functions from each lib and the GUID is created properly
+    wic_main="
+      int main(void) {
+        CLSID_WICImagingFactory;
+        CoInitialize(NULL);
+        SHCreateStreamOnFile(NULL, 0, NULL);
+        return 0;
+      }
+      "
+    AC_LANG_PUSH(C)
+    AC_LINK_IFELSE(
+      [AC_LANG_SOURCE([
+         $wic_headers
+         $wic_main])],
+      [wic_support=yes],
+      [wic_support=no]
+    )
+    AC_LANG_POP
+
+    test "$wic_support" = "yes" || LIBS=$saved_LIBS
+    AC_MSG_RESULT(${wic_support-no})
+  fi
+fi
+
 dnl === If --enable-experimental is defined, add the flag WEBP_EXPERIMENTAL_FEATURES
 
 USE_EXPERIMENTAL_CODE=""