__acrt_iob_func() -> __iob_func() on Appveyor

Works around rust-lang/rust#47048 via workaround described in nabijaczleweli/cargo-update#74.

.appveyor.yml

@@ -12,6 +12,11 @@ install:
   - rust-nightly-%TARGET%.exe /VERYSILENT /NORESTART /DIR="C:\Program Files (x86)\Rust"
   - set PATH=%PATH%;C:\Program Files (x86)\Rust\bin
   - if defined MSYS_BITS set PATH=%PATH%;C:\msys64\mingw%MSYS_BITS%\bin
+  -
+  # Fix undefined references to __acrt_iob_func() per
+  # https://github.com/nabijaczleweli/cargo-update/issues/74#issuecomment-376581009
+  - sed -rie "s/#define std([[:alpha:]]+)[[:space:]]+\(__acrt_iob_func\(([[:digit:]]+)\)\)/#define std\1 (\&__iob_func()[\2])/" "C:\msys64\mingw64\x86_64-w64-mingw32\include\stdio.h"
+  -
   - rustc -V
   - cargo -V