+20190623
+ + improve the tabs.1 manual page to distinguish the PWB/Unix and 7th
+ Edition versions of the tabs utility.
+ + add configure check for getenv() to work around implementation shown
+ in Emscripten #6766, use that to optionally suppress START_TRACE
+ macro, whose call to getenv() may not work properly (report by Ilya
+ Ig Petrov).
+ + modify initialization functions to avoid relying upon persistent
+ data for the result from getenv().
+ + update config.guess, config.sub
+