nginx: change how the perl module is configured

Previously, http_perl_module was disabled by overriding perl=null -- this means
it is impossible to disable http_perl_module in openresty, since openresty
requires perl for its configure scripts.

pkgs/servers/http/nginx/generic.nix

@@ -4,6 +4,7 @@
 , withDebug ? false
 , withStream ? true
 , withMail ? false
+, withPerl ? true
 , modules ? []
 , ...
 }:
@@ -87,7 +88,7 @@ stdenv.mkDerivation {
   ] ++ optionals withMail [
     "--with-mail"
     "--with-mail_ssl_module"
-  ] ++ optional (perl != null) [
+  ] ++ optionals withPerl [
     "--with-http_perl_module"
     "--with-perl=${perl}/bin/perl"
     "--with-perl_modules_path=lib/perl5"

pkgs/top-level/all-packages.nix

@@ -15847,14 +15847,14 @@ in
   nginx = nginxStable;
 
   nginxStable = callPackage ../servers/http/nginx/stable.nix {
-    perl = null;
+    withPerl = false;
     # We don't use `with` statement here on purpose!
     # See https://github.com/NixOS/nixpkgs/pull/10474/files#r42369334
     modules = [ nginxModules.rtmp nginxModules.dav nginxModules.moreheaders ];
   };
 
   nginxMainline = callPackage ../servers/http/nginx/mainline.nix {
-    perl = null;
+    withPerl = false;
     # We don't use `with` statement here on purpose!
     # See https://github.com/NixOS/nixpkgs/pull/10474/files#r42369334
     modules = [ nginxModules.dav nginxModules.moreheaders ];