mosh: add withClient to allow perlless server-only builds (#474070)

pkgs/by-name/mo/mosh/package.nix

@@ -15,6 +15,8 @@
   fetchpatch,
   withUtempter ? stdenv.hostPlatform.isLinux && !stdenv.hostPlatform.isMusl,
   libutempter,
+  # build server binary only when set to false (useful for perlless systems)
+  withClient ? true,
 }:
 
 stdenv.mkDerivation rec {
@@ -41,6 +43,8 @@ stdenv.mkDerivation rec {
     zlib
     openssl
     bash-completion
+  ]
+  ++ lib.optionals withClient [
     perl
   ]
   ++ lib.optional withUtempter libutempter;
@@ -70,9 +74,17 @@ stdenv.mkDerivation rec {
 
   configureFlags = [ "--enable-completion" ] ++ lib.optional withUtempter "--with-utempter";
 
-  postInstall = ''
-    wrapProgram $out/bin/mosh --prefix PERL5LIB : $PERL5LIB
-  '';
+  postInstall =
+    if withClient then
+      ''
+        wrapProgram $out/bin/mosh --prefix PERL5LIB : $PERL5LIB
+      ''
+    else
+      ''
+        rm $out/bin/mosh
+        rm $out/bin/mosh-client
+        rm -r $out/share/{man,bash-completion}
+      '';
 
   meta = {
     homepage = "https://mosh.org/";