At every place where an "$extension" is used, include the leading

At every place where an "$extension" is used, include the leading
dot in the extension, don't hardcode it when building the filename.
That will make any support for a dot-less extension easier to add.

* automake.in (SUFFIX_RULE_PATTERN): Include extension dots in
backref groups.
("main"): Prepend a '.' to the supported extensions of all
languages (e.g. 'c' becomes '.c').
(handle_languages, handle_single_transform_list,
handle_lib_objects_cond, handle_headers, derive_suffix,
rule_define): Do not add a dot
before extensions, and include dots in matching groups.
* lib/am/depend2.am (.%EXT%.o, .%EXT%.obj, .%EXT%.lo): Rename as ...
(%EXT%.o, %EXT%.obj, %EXT%.lo): ... these.
* lib/am/lex.am (.%EXT%.%DERIVED-EXT%): Rename as ...
(%EXT%%DERIVED-EXT%): ... this.
* lib/am/yacc.am: Likewise.