genhelp: improve to work with lua 5.1

Fix the script to not contain \x00 escape which was the only new
feature used.

Fix the meson build script to use given lua interpreter for running
the genhelp.lua.

Based on patch from Daniel Golle <daniel@makrotopia.org>

src/genhelp.lua

@@ -198,12 +198,12 @@ end
 function scdoc:render_optgroups(out)
 	for _, options in ipairs(self.optgroup) do
 		if #options > 0 then
-			table.insert(out, options.name .. "\x00")
+			table.insert(out, options.name .. "\0")
 			self:render_options(out, options)
 			if options.name == self.applet then
 				self:render_footer(out)
 			end
-			table.insert(out, "\x00")
+			table.insert(out, "\0")
 		end
 	end
 end
@@ -216,7 +216,7 @@ function scdoc:render(out)
 	local width = self.width
 
 	if not self.applet then return end
-	table.insert(out, self.applet .. "\x00")
+	table.insert(out, self.applet .. "\0")
 	table.insert(out, table.concat(self.usage, "\n"))
 	table.insert(out, "\n")
 	if #self.commands > 0 then
@@ -239,7 +239,7 @@ function scdoc:render(out)
 		if self.manpage == "apk" then self:render_footer(out)
 		else table.insert(out, "\n") end
 	end
-	table.insert(out, "\x00")
+	table.insert(out, "\0")
 end
 
 local function compress(data)
@@ -293,7 +293,7 @@ local out = {}
 for _, doc in ipairs(f) do doc:render(out) end
 for _, doc in ipairs(f) do doc:render_optgroups(out) end
 
-table.insert(out, "\x00")
+table.insert(out, "\0")
 
 local help = table.concat(out)
 --io.stderr:write(help)

src/meson.build

@@ -78,14 +78,14 @@ apk_src = [
 ]
 
 if lua_bin.found()
-	genhelp_script = find_program('./genhelp.lua')
+	genhelp_script = files('genhelp.lua')
 
 	generated_help = custom_target(
 		'help.h',
 		capture: true,
 		output: 'help.h',
 		input: man_files,
-		command: [genhelp_script, '@INPUT@'],
+		command: [lua_bin, genhelp_script, '@INPUT@'],
 	)
 else
 	generated_help = custom_target(