Fix comments style in configure.ac and Makefile.am

Fix comments style in configure.ac and Makefile.am
(code refactoring)

Do not allow the comments that are meaningless in the auto-generated
configure and Makefile.in to be put to these files.

* Makefile.am (EXTRA_DIST, SINGLE_GC_OBJ, EXTRA_DIST,
include doc/doc.am): Change comment style from "#" to "##".
* Makefile.am (CPLUSPLUS): Move "#" inside the conditional block.
* configure.ac: Update the copyright.
* configure.ac: Change comment style from "#" to "dnl" for the comments
that are meaningless in auto-generated configure file.