[Patch] dbus sysconfdir default value

[Timothée Lecomte <timothee lecomte ens fr>]

Dear NetworkManager developpers,

In the process of compiling and installing NetworkManager on my computer
running a non-recognised distribution (Sourcemage,
http://www.sourcemage.org), I encountered a few obstacles that I solved by
hand, mostly installation issues. I will try to provide patches for them.

Here is the first one :

When you don't provide --with-dbus-sys=***, the ./configure script chooses
${sysconfdir}/dbus-1/system.d

This is almost always wrong. On most distributions, it should be
/etc/dbus-1/system.d whereas the previous code expands to
$prefix/etc/dbus-1/system.d where $prefix is often /usr, or whatever is
given to ./configure.

However, the dbus package (since 0.60 which is the minimum version for NM)
provides this information through pkg-config, so let's use it !

See attached patch.

Thank you for considering comitting this.

Best regards,

Timothée Lecomte