Set configure defaults from --enable-developer, not Automake maintainer mode

Set configure defaults from --enable-developer, not Automake maintainer mode

Automake maintainer mode isn't about whether you're a maintainer or not
(although its name would suggest that), it's about whether files that are
normally distributed in the tarball get regenerated. As such, it's
not really appropriate to use it to drive defaults for things like
assertions and extra test code.

The desired effect is that developers building from git normally get
tests and assertions, while distribution packagers don't.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=34671
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Colin Walters <walters@verbum.org>