build-sys: allow building man pages even if disabled in ./configure

This is purely for developer convenience: building man pages is slow
so people tend to disable them when not working on documentation. But
sometimes it is useful to build the man page to look something up,
especially when working on an older machine which has outdated
documentation, or to test some change to documentation. The rules to build
man pages are now outside of the ENABLE_MANPAGES conditional, but they
are not used unless manually invoked, and only the inclusion of man
pages in build and install targets is affected by the conditional.

Also, more generated files are removed in clean, which seems to be
good thing, and Makefile.am is simplified.

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