build: silence warning from autoconf

build: silence warning from autoconf

Autoconf 2.60 and later insist on using ${datarootdir}, rather than
the derived ${datadir} (although the latter defaults to the former,
it is possible to set configure arguments so that they differ):

config.status: creating libvirt.pc
config.status: WARNING:  'libvirt.pc.in' seems to ignore the --datarootdir setting

This patch follows the autoconf manual's suggestions for how to
support 2.59 (RHEL 5) and newer simultaneously.

* libvirt.pc.in (datarootdir): Define, so ${datadir} will not ignore
datarootdir when using newer autoconf.