refactor: new variable telling whether make is running in "dry mode"

refactor: new variable telling whether make is running in "dry mode"

We define a new variable '$(am__make_dryrun)', that can be used
in recipes to determine whether make is running in "dry mode" (as
with "make -n") or not.  This is useful in rules that invoke make
recursively, and are thus executed also with "make -n" -- either
because they are declared as dependencies to '.MAKE' (NetBSD make)
or because their recipes contain the "$(MAKE)" string (GNU and
Solaris make).

* lib/am/header-vars.am (am__make_dryrun): New variable.
* lib/am/check.am (check-TESTS recheck): Use it, and simplify
recipe accordingly.
* lib/am/distdir.am (distdir): Likewise.
* lib/am/lisp.am ($(am__ELCFILES)): Likewise.