configure.ac: Make automake shut up about subdir-objects

Not only that though; In AutoMake 2.0, the subdir-objects option
will apparently be turned on by default. Some projects have
reported hard-to-solve bugs when enabling the option to get rid
of warnings. We're not getting any, so it should be reasonably
safe to turn it on.

Signed-off-by: Andreas Ericsson <ae@op5.se>
configuration: Don't segfault on unavailable include_dirs

The return value was never checked, and hence an unavailable include_dir
resulted in naemon happily trying to ask for the next dirent in the DIR
NULL. This didn't turn out so well.

Change-Id: Iea9676e5b2d9881a6285b0405421b4bc4aca50aa
Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>