build: the user can override AUTOM4TE, AUTORECONF and AUTOUPDATE too

build: the user can override AUTOM4TE, AUTORECONF and AUTOUPDATE too

Our build system allows the user to override AUTOCONF and AUTOHEADER
at configure time, and honours these overrides in our testsuite.
But it didn't do the same with AUTOM4TE, AUTORECONF and AUTOUPDATE.
This change fixes that inconsistency.

* configure.ac (am_AUTOM4TE, am_AUTOUPDATE, am_AUTORECONF): New
AC_SUBSTitutions.  Update comments.
* tests/defs.in ($AUTOUPDATE): Default to `@am_AUTOUPDATE@' now.
($AUTOM4TE): New variable, defaulting to `@am_AUTOM4TE@'.
($AUTORECONF): New variable, defaulting to `@am_AUTORECONF@'.
* doc/Makefile.am ($(srcdir)/amhello-1.0.tar.gz): Update.