Add support for reference links

https://bugzilla.gnome.org/show_bug.cgi?id=723812
tests: fix broken image
tests: add testcase for <http://foo.com> style links
tests: add another test for uris

This throws lots of errors unless one adds at least a single char between '*' and '<'.
make: remove the types file on clean if we generated it
Use proper quotation marks
Don't allow < in ref link targets

Fixes build of GTK+ 3.11.5
Don't interpret <http://foo.com> as markup

Let it fall into a paragraph and parse it as a link.
gtk-doc.pc: keep datda-dir relocatable
scangobj: use EXEEXT to make the scanner work on platforms such as windows
make: Don't fail on removing types files if they don't exist

This fixes make dist, broken by d5110ef6d1825fc1f4f7f7bcfe149e48ede972f8
mkdb: Prevent duplicate IDs in boxed union types

When registering a C union as a GBoxed type, Gtk-Doc generates
duplicate IDs. This handles the problem in the same way as structs
handle it.

Fixes bgo#669417.
mkdb: fix CheckIsObject return value

It needs brackets around the return expression or it doesn't seem to evaluate
the "$root ne 'GEnum' and $root ne 'GFlags'" bit.
Spotted by Damon Chaplin

Fixes #706404
build: make distcehck fixes
Always wrap blocks in list items in para

We were only wrapping the last item in a para.
Which results in invalid docbook and causes
inconsistent list items.

https://bugzilla.gnome.org/show_bug.cgi?id=723913
Correctly handle neighboring ordered and unordered lists
Also handle <https:> tags
mkdb: don't hardcode https?, but handle all schemas according to rfc2396
docs: mention DISTCHECK_CONFIGURE_FLAGS in the manual

Explain how to ensure that make dist will include prebuild docs.
docs: update the docs for 1.20

Add a section 'modernizing' that helps to decide which verison to require for which feature.
Expand the markdown support a little.