Fixed a bug where systemd unit file couldn't be installed

Fixed a bug where systemd unit file couldn't be installed
because PKG_PROG_PKG_CONFIG macro wasn't expanded in configure
script.

Mail from Peter Schiffer <pschiffe@redhat.com> 08/03/2013:

I'm writing you regarding a little problem I've noticed when running
./configure script:

./configure: line 3923: PKG_PROG_PKG_CONFIG: command not found
checking for systemctl... /bin/systemctl
./configure: line 3975: --variable=systemdsystemunitdir: command not found

According to Google, you might be missing pkg-config program in your
path while generating configure script from configure.in file.
Because of this, systemd unit file won't be installed while doing make
install.