configure: Support using custom builds of elfutils

configure: Support using custom builds of elfutils

It should be possible to build and install a custom elfutils (e.g,
from its source repository) under a particular prefix and have
libabigail use that custom elfutils.

This patch adds that capability by adding these two new options:
--with-libelf=/path/to/libelf/prefix and
--with-libdw=/path/to/libdw/prefix.

	* configure.ac: Add support for --with-libdw and --with-libelf.
	Use pkg-config to detect the presence of libdw and libelf either
	on the system or below the prefixes specified by the new
	aforementionned options.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>