appveyor: Install autotools package on 2019 image

On the recently updated 2019 image, autoreconf is not found anymore, as
recent versions of msys2 don't ship autools with base-devel aymore, so
install the autotools package explicitly.

.appveyor.yml

@@ -30,6 +30,8 @@ install:
       IF "%IMG%" == "2019" set OPENSSL=OpenSSL-v111
       set OPENSSL_DIR=/c/%OPENSSL%-%TEST%
       C:\%OPENSSL%-%TEST%\bin\openssl.exe version -a
+  # newer versions of msys2 don't provide autotools via base-devel anymore
+  - IF "%IMG%" == "2019" %MSYS_SH% --login -c ". /etc/profile && pacman --noconfirm -S --needed autotools"
 
 build_script:
   - '%MSYS_SH% --login -c ". /etc/profile && cd $APPVEYOR_BUILD_FOLDER && ./scripts/test.sh deps"'