civetweb: enable IPv6

It's disabled by default...

This is necessary for prometheus-cpp to use IPv6 addresses (e.g.
`[::]:9999`).

pkgs/development/libraries/civetweb/default.nix

···
   '';
 
   cmakeFlags = [
+    "-DBUILD_SHARED_LIBS=ON"
     "-DCIVETWEB_ENABLE_CXX=ON"
-    "-DBUILD_SHARED_LIBS=ON"
+    "-DCIVETWEB_ENABLE_IPV6=ON"
 
     # The civetweb unit tests rely on downloading their fork of libcheck.
     "-DCIVETWEB_BUILD_TESTING=OFF"