page.tmpl: tell mobile browsers we have a responsive layout, unless told not to

Mobile browsers typically assume that arbitrary web pages are
designed for a "desktop-sized" browser window (around 1000px)
and display that layout, zoomed out, in order to avoid breaking
naive designs that assume nobody will ever look at a website on
a phone or something. People who are actually doing "responsive
design" need to opt-in to mobile browsers rendering it at a
more normal size.

IkiWiki.pm

@@ -549,6 +549,13 @@ sub getsetup () {
 		safe => 0,
 		rebuild => 0,
 	},
+	responsive_layout => {
+		type => "boolean",
+		default => 1,
+		description => "theme has a responsive layout? (mobile-optimized)",
+		safe => 1,
+		rebuild => 1,
+	},
 }
 
 sub defaultconfig () {

IkiWiki/Render.pm

@@ -134,6 +134,7 @@ sub genpage ($$) {
 		ctime => displaytime($pagectime{$page}, undef, 1),
 		baseurl => baseurl($page),
 		html5 => $config{html5},
+		responsive_layout => $config{responsive_layout},
 	);
 
 	run_hooks(pagetemplate => sub {

templates/page.tmpl

@@ -8,6 +8,7 @@
 </TMPL_IF>
 <TMPL_IF HTML5><meta charset="utf-8" /><TMPL_ELSE><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></TMPL_IF>
 <title><TMPL_VAR TITLE></title>
+<TMPL_IF RESPONSIVE_LAYOUT><meta name="viewport" content="width=device-width, initial-scale=1" /></TMPL_IF>
 <TMPL_IF FAVICON>
 <link rel="icon" href="<TMPL_VAR BASEURL><TMPL_VAR FAVICON>" type="image/x-icon" />
 </TMPL_IF>