configure: Add pkgconfig file

Thus, path discovery becomes mostly a non-issue.

This opts to include the name of the fork in the include path, to
encourage users to include the header itself, without the directory
named after the fork in every include - i.e. #include <objects.h> rather
than #include <naemon/objects.h>. This way, the whole "drop-in
replacement" spiel is still just mildly suspect, rather than the fucking
joke it would be if module authors were encouraged to include the fork
name in every single include path, the way some modules - such as merlin
- currently does. Using this will still not "just work" across both,
because nagios does not have this commit as far as I know, but at least
some differences can be papered over build-time, if the user would want
to.

Signed-off-by: Robin Sonefors <ozamosi@flukkost.nu>