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The correct option's name is "warning_level", so change it for it
to actually be effective while building.

README, doap: replace mailing list reference with Discourse generic applications category Infrastructure/Infrastructure#454

<item> cannot include <title> elements. Use <p> elements instead.

This makes sense for dev builds but can potentially break the build
in environment where any library frogr depends on is building with
warnings, as that would of course break the build.

This has been recently the case for Hurd and FreeBSD (detected by
Debian), so let's prevent thi from happening by setting saner defaults.

No need to check for GTK+ >= 3.12 when enabling the header bar
when the minimum version for GTK+ is 3.16 in any case.

They are implicit in the latest runtimes anyway.

Frogr doesn't use GSettings at all, so this is not really needed.

GTimeVal has been deprecated since version 2.62 due to not being
year-2038-safe, so let's migrate away from it now.