zeromq: optional libsodium dependency

Closes Homebrew/homebrew#23180.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/zeromq.rb

@@ -19,6 +19,7 @@ class Zeromq < Formula
 
   depends_on 'pkg-config' => :build
   depends_on 'libpgm' if build.include? 'with-pgm'
+  depends_on 'libsodium' => :optional
 
   def install
     ENV.universal_binary if build.universal?
@@ -31,6 +32,8 @@ class Zeromq < Formula
       args << "--with-system-pgm"
     end
 
+    args << "--with-libsodium" if build.with? 'libsodium'
+
     system "./autogen.sh" if build.head?
     system "./configure", *args
     system "make"