Also makes it configurable via configure script. Depending on `$datadir` is
not ideal as package maintainers might set that to a custom value. Depending
on `$datarootdir` might have been better, the default if pkg-config fails is
now based on that.
References #3339.
Since D-Bus 1.9.18 configuration files installed by third-party should
go in share/dbus-1/system.d. The old location is for sysadmin overrides.
Closesstrongswan/strongswan#150.
INCLUDES are now deprecated and throw warnings when using automake 1.13.
We now also differentiate AM_CPPFLAGS and AM_CFLAGS, where includes and
defines are passed to AM_CPPFLAGS only.