am: stop using "%KEY:iffalse?iftrue%" transforms

am: stop using "%KEY:iffalse?iftrue%" transforms

And the similar "reduced forms" "%KEY:iffalse%" and "%KEY?iftrue%"
as well.

They are harder to grasp, can be easily reformulated in function of
other "plainer" transforms ("?KEY?" and "?!KEY?"), and we'll remove
support for them anyway in a later change.

* lib/am/data.am, lib/am/libs.am, lib/am/lisp.am, lib/am/ltlib.am,
lib/am/progs.am, lib/am/python.am, lib/am/scripts.am: Adjusted not
to rely on those kinds of transforms.

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