Add a shared_target to the VC-common config

This has no real meaning, except it gives Configure a hint that VC
targets are indeed capable of producing shared objects.

Reviewed-by: Matt Caswell <matt@openssl.org>

Configurations/10-main.conf

@@ -1104,6 +1104,7 @@
         template         => 1,
         cc               => "cl",
         cflags           => "-W3 -wd4090 -Gs0 -GF -Gy -nologo -DOPENSSL_SYS_WIN32 -DWIN32_LEAN_AND_MEAN -DL_ENDIAN -D_CRT_SECURE_NO_DEPRECATE",
+        shared_target    => "win-shared", # meaningless except it gives Configure a hint
         dso_scheme       => "win32",
 	build_scheme     => "mk1mf",
     },