[ng] maintcheck: refactor checks on obsolete variable names

[ng] maintcheck: refactor checks on obsolete variable names

This is just a preparatory refactoring, to make future additions
easier.

* syntax-checks.mk (sc_renamed_variables_rules): New autocomputed
variable.
($(sc_renamed_variables_rules)): New static pattern rule, subsuming
most of the existing checks against the use of old names for some
automake provided make variables.
(syntax_check_rules): Add the contents of the new variable, and do
not explicitly list the checks subsumed by it.

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