[ng] refactor: merge inst-vars.am into header-vars.mk

[ng] refactor: merge inst-vars.am into header-vars.mk

This will simplify possible future refactorings.  Given the small size
of 'inst-vars.am', and the fact that the content of a verbatim-included
makefile fragment is included in the distribution tarball only once,
this change doesn't increase the size of distribution tarballs in any
appreciable way, even if the variables once defined in 'inst-vars.am'
aren't really needed.

* lib/am/inst-vars.am: Remove, merged ...
* lib/am/header-vars.mk: ... in here, with all '##' comments turned
into '#' comments, and (few of them) slightly improved with the
addition of "FIXME" remarks.
* Makefile.am (dist_am_DATA): Adjust.
* lib/am/data.am: Include the 'inst-vars.am' file explicitly no more.
* lib/am/libs.am:: Likewise.
* lib/am/lisp.am:: Likewise.
* lib/am/ltlib.am:: Likewise.
* lib/am/mans.am:: Likewise.
* lib/am/python.am:: Likewise.
* lib/am/scripts.am:: Likewise.
* lib/am/texinfos.am:: Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>