examples: Remove outdated variables from example Makefile.am

Everyone has automake 1.6 now. Specifying EXTRA_DIST is a bit pointless.

https://bugzilla.gnome.org/show_bug.cgi?id=741763
examples: Enable gtk-doc `make check` tests by default

Otherwise they never get enabled. If people have problems getting the
tests to pass, they should either fix their documents, or we should make
the tests cleverer.

https://bugzilla.gnome.org/show_bug.cgi?id=741763
make: Automatically clean sections.txt if --rebuild-sections is used

Rather than documenting it in a comment in the example Makefile.am;
might as well automate it.

https://bugzilla.gnome.org/show_bug.cgi?id=741763
make: Use $(expand_content_files) wherever $(content_files) is used

This means we can eliminate the advice that all $(expand_content_files)
entries must also be in $(content_files), simplifying people’s makefiles
a little.

https://bugzilla.gnome.org/show_bug.cgi?id=741763