configure: force -fno-strict-aliasing for C++ modules using COM objects

This avoids potential compiler optimizations when casting to void** when using
IID_PPV_ARGS or WRL.

If the option is not supported we fail the configure as many important C++
modules may not work as expected due to pointer aliasing. In practice this
should never happen as GCC for mingw-w64 and Clang support this correctly.

configure.ac

@@ -388,6 +388,16 @@ AC_ARG_ENABLE([winstore_app],
 vlc_winstore_app=0
 AS_IF([test "${SYS}" = "mingw32"],[
   LIBCOM="-lole32 -loleaut32"
+  AC_LANG_PUSH(C++)
+  AX_CHECK_COMPILE_FLAG([-fno-strict-aliasing], [
+        LIBCOMCXXFLAGS="-fno-strict-aliasing"
+    ],[
+        # fno-strict-aliasing is necessary for WRL and IID_PPV_ARGS to work safely
+        # MSVC doesn't have this option but doesn't do pointer aliasing, so it
+        # should work too
+        AC_MSG_ERROR([-fno-strict-aliasing is necessary for Windows C++ modules])
+    ])
+  AC_LANG_POP(C++)
   AS_IF([test "${enable_winstore_app}" = "yes"], [
     vlc_winstore_app=1
     LIBCOM="-loleaut32"
@@ -400,6 +410,7 @@ AS_IF([test "${SYS}" = "mingw32"],[
     AC_DEFINE([VLC_WINSTORE_APP], [1], [Define to 1 if building for Windows Store.])
   ],[])
   AC_SUBST([LIBCOM])
+  AC_SUBST([LIBCOMCXXFLAGS])
   ])
 
 vlc_build_pdb=0