mklldeps: Don't link stdc++/c++ on MSVC

These libraries don't exist! The linker for MSVC is highly likely to not pass
`/NODEFAULTLIB` in which case the right standard library will automatically be
selected.

Author: alexcrichton

src/etc/mklldeps.py

@@ -80,10 +80,13 @@ def run(args):
     assert('stdlib=libc++' not in out)
     f.write("#[link(name = \"stdc++\", kind = \"static\")]\n")
 else:
+    # Note that we use `cfg_attr` here because on MSVC the C++ standard library
+    # is not c++ or stdc++, but rather the linker takes care of linking the
+    # right standard library.
     if 'stdlib=libc++' in out:
-        f.write("#[link(name = \"c++\")]\n")
+        f.write("#[cfg_attr(not(target_env = \"msvc\"), link(name = \"c++\"))]\n")
     else:
-        f.write("#[link(name = \"stdc++\")]\n")
+        f.write("#[cfg_attr(not(target_env = \"msvc\"), link(name = \"stdc++\"))]\n")
 
 # Attach everything to an extern block
 f.write("extern {}\n")