automake: refactor pre-processing of makefile fragments This change will provide the automake script with a new function that reads in a Makefile fragment *without* performing Automake ad-hoc parsing, but only the pre-processing step, i.e., removal of '##' comments and substitution of tokens like '%SUBDIRS%', '%?LIBTOOL%' or '?GENENRIC?'. This will very likely be useful for the work on Automake-NG. This is a pure refactoring, with no intended functional or semantic changes. * automake.in (preprocess_file): New function, extracted ... (make_paragraphs): ... from here. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>