am: make function to canonicalize names

am: make function to canonicalize names

We implement a make function that canonicalizes names the same way the
automake script does (with the '&canonicalize' perl function):

    sub/prog      =>  sub_prog
    libx-old.a    =>  libx_old_a
    devel/libx.a  =>  devel_libx_a

This new make function is still unused in our codebase, but it's nice
to have it ther,e ready and tested, should the need for it ever arise.

* lib/am/header-vars.am (am__canon): New function.
(am__bslash, am__comma, am__dollar, am__pound, am__lparen, am__rparen,
am__bquote, am__dquote, am__squote ): New immediate variables, used by
the above function to avoid spurious syntax errors (e.g., $(am__comma)
expands to ',', which would be unusable directly in a make function call).
* t/internals.tap: Extend.

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