Appveyor Windows 64-bit builds fail because the compiler is broken

Since about 6 hours ago, all Appveyor 64-bit Windows builds fail the first C file they build with:

bash.exe : In file included from ../src/core/or/or.h:16,
At line:2 char:5
+     & $commandPath $args 2>&1
+     ~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (In file include...ore/or/or.h:16,:String) [], RemoteException
    + FullyQualifiedErrorId : NativeCommandError
 
                 from ../src/app/main/ntmain.c:22:
../src/lib/cc/torint.h:52:2: error: #error "Seems that your platform doesn't use 2's complement arithmetic. Argh."
 #error "Seems that your platform doesn't use 2's complement arithmetic. Argh."
  ^~~~~
In file included from ../src/core/or/or.h:26,
                 from ../src/app/main/ntmain.c:22:
../src/lib/cc/compat_compiler.h:28:2: error: #error "It seems your platform does not represent NULL as zero. We can't cope."
 #error "It seems your platform does not represent NULL as zero. We can't cope."
  ^~~~~
../src/lib/cc/compat_compiler.h:32:2: error: #error "It seems your platform does not represent 0.0 as zeros. We can't cope."
 #error "It seems your platform does not represent 0.0 as zeros. We can't cope."
  ^~~~~
In file included from ../src/core/or/or.h:16,
                 from ../src/feature/dirauth/dircollate.h:16,
                 from ../src/feature/dirauth/dircollate.c:25:
../src/lib/cc/torint.h:52:2: error: #error "Seems that your platform doesn't use 2's complement arithmetic. Argh."
 #error "Seems that your platform doesn't use 2's complement arithmetic. Argh."
  ^~~~~
In file included from ../src/core/or/or.h:26,
                 from ../src/feature/dirauth/dircollate.h:16,
                 from ../src/feature/dirauth/dircollate.c:25:
../src/lib/cc/compat_compiler.h:28:2: error: #error "It seems your platform does not represent NULL as zero. We can't cope."
 #error "It seems your platform does not represent NULL as zero. We can't cope."
  ^~~~~
../src/lib/cc/compat_compiler.h:32:2: error: #error "It seems your platform does not represent 0.0 as zeros. We can't cope."
 #error "It seems your platform does not represent 0.0 as zeros. We can't cope."
  ^~~~~

https://ci.appveyor.com/project/teor2345/tor/build/1.0.113/job/ujbvwntcu1pdk2m6#L762

CC'ing mikeperry, because he's on CI rotation this week. Which is weird, because configure seems to succeed.