Force epoch for typesetting (see #9)

At present, this is modelled with the idea that an epoch at the
command line forces for both check and doc. We could have

    texlua build.lua doc -E <date> -f

so you also have to explicitly force: seems over-the-top to
me.

l3build.dtx

@@ -136,6 +136,9 @@
 \luavarset{makeindexexe} {"makeindex"}{MakeIndex executable}
 \luavarset{makeindexopts}{""}         {MakeIndex options}
 \luavarseparator
+\luavarset{forcecheckepoch}{"true"}    {Force epoch when running tests}
+\luavarset{forcedocepoch}  {"false"}   {Force epoch when typesetting}
+\luavarseparator
 \luavarset{asciiengines}{\{"pdftex"\}}{Engines which should log as sure ASCII}
 \luavarset{checkruns}   {1}           {How many times to run a check file before comparing the log.}
 \luavarset{epoch}       {1463734800}  {Epoch (Unix date) to set for test runs.}

l3build.lua

@@ -159,6 +159,14 @@ bibtexopts    = bibtexopts    or "-W"
 makeindexexe  = makeindexexe  or "makeindex"
 makeindexopts = makeindexopts or ""
 
+-- Forcing epoch
+if forcecheckepoch == nil then
+  forcecheckepoch = true
+end
+if forcedocepoch == nil then
+  forcedocepoch = true
+end
+
 -- Other required settings
 asciiengines = asciiengines or {"pdftex"}
 checkruns    = checkruns    or 1
@@ -1392,13 +1400,14 @@ function runtest(name, engine, hide, ext, makepdf)
       -- Avoid spurious output from (u)pTeX
       os_setenv .. " GUESS_INPUT_KANJI_ENCODING=0"
         .. os_concat ..
-      -- Fix the time of the run
-      os_setenv .. " SOURCE_DATE_EPOCH=" .. epoch
-        .. os_concat ..
-      os_setenv .. " SOURCE_DATE_EPOCH_TEX_PRIMITIVES=1"
-        .. os_concat ..
-      os_setenv .. " FORCE_SOURCE_DATE=1"
-        .. os_concat ..
+      if forcetestepoch then
+        os_setenv .. " SOURCE_DATE_EPOCH=" .. epoch
+          .. os_concat ..
+        os_setenv .. " SOURCE_DATE_EPOCH_TEX_PRIMITIVES=1"
+          .. os_concat ..
+        os_setenv .. " FORCE_SOURCE_DATE=1"
+          .. os_concat ..
+      end
       -- Ensure lines are of a known length
       os_setenv .. " max_print_line=" .. maxprintline
         .. os_concat ..
@@ -1496,7 +1505,15 @@ function runtool(subdir, dir, envvar, command)
   return(
     run(
       typesetdir .. "/" .. subdir,
-      os_setenv .. " " .. envvar .. "=." .. os_pathsep
+        if forcedocepoch then
+          os_setenv .. " SOURCE_DATE_EPOCH=" .. epoch
+            .. os_concat ..
+          os_setenv .. " SOURCE_DATE_EPOCH_TEX_PRIMITIVES=1"
+            .. os_concat ..
+          os_setenv .. " FORCE_SOURCE_DATE=1"
+            .. os_concat ..
+        end
+        os_setenv .. " " .. envvar .. "=." .. os_pathsep
         .. abspath(localdir) .. os_pathsep
         .. abspath(dir .. "/" .. subdir)
         .. (typesetsearch and os_pathsep or "") ..