Fix d2d1.h detection: C++ is required

configure.ac

@@ -3334,18 +3334,21 @@ dnl
 dnl  Windows Direct2D plugin
 dnl
 AC_ARG_ENABLE(direct2d,
-  [  --enable-direct2d         Win7/VistaPU Direct2D support (default auto on Win32)])
-if test "${enable_direct2d}" != "no"; then
-  if test "${SYS}" = "mingw32" -o "${SYS}" = "mingwce"
-  then
-    AC_CHECK_HEADERS(d2d1.h,
-      [
+  [  --enable-direct2d         Win7/VistaPU Direct2D support (default auto on Win32)],, [
+  AS_IF([test "${SYS}" != "mingw32" -a "${SYS}" != "mingwce"], [
+    enable_direct2d="no"
+  ])
+])
+AS_IF([test "${enable_direct2d}" != "no"], [
+  AC_LANG_PUSH([C++])
+  AC_CHECK_HEADERS(d2d1.h, [
     VLC_ADD_PLUGIN([direct2d])
     VLC_ADD_LIBS([direct2d],[-lgdi32 -lole32])
-      ], [AC_MSG_WARN([Cannot find Direct2D headers!])]
-    )
-  fi
-fi
+  ], [
+    AC_MSG_WARN([Cannot find Direct2D headers!])
+  ])
+  AC_LANG_POP([C++])
+])
 
 dnl
 dnl  win32 GDI plugin