Build "more compatible" executables on AppVeyor

Pass the `-C target-feature=+crt-static` feature on AppVeyor to ensure that
binaries produced on Windows have as few dynamic dependencies as possible,
making them more portable to run on more Windows machines.

.appveyor.yml

@@ -1,6 +1,10 @@
+environment:
+  global:
+    RUSTFLAGS: -C target-feature=+crt-static
+
 install:
   - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
-  - if not defined RUSTFLAGS rustup-init.exe -y --default-host x86_64-pc-windows-msvc --default-toolchain nightly
+  - rustup-init.exe -y --default-host x86_64-pc-windows-msvc --default-toolchain nightly
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
   - rustc -V
   - cargo -V