nginx: Fix cross-OS build

nginx uses uname to find the target platform by default.
This works fine when building locally or when building on the same OS
for most architectures, but breaks when building e.g. Linux to BSD.

Explicitly tell the target to the configure script instead.

pkgs/servers/http/nginx/generic.nix

@@ -170,6 +170,9 @@ stdenv.mkDerivation {
       [ "--with-http_geoip_module" ] ++ lib.optional withStream "--with-stream_geoip_module"
     )
     ++ lib.optional (with stdenv.hostPlatform; isLinux || isFreeBSD) "--with-file-aio"
+    ++ lib.optional (
+      stdenv.buildPlatform != stdenv.hostPlatform
+    ) "--crossbuild=${stdenv.hostPlatform.uname.system}::${stdenv.hostPlatform.uname.processor}"
     ++ configureFlags
     ++ map (mod: "--add-module=${mod.src}") modules;