brotli: ensure CMAKE_SYSTEM_NAME=Windows on mingw cross builds

CMAKE passes the flag -rdynamic otherwise (which is not recognized by mingw).
Setting the appropriate flag avoids this problem and allows the build
to succeed.

pkgs/tools/compression/brotli/default.nix

@@ -15,6 +15,9 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ cmake ];
 
+  cmakeFlags = stdenv.lib.optional
+    (stdenv.hostPlatform.libc == "msvcrt") "-DCMAKE_SYSTEM_NAME=Windows";
+
   outputs = [ "out" "dev" "lib" ];
 
   doCheck = true;