package/libglib2: add optional support for elfutils

package/libglib2: add optional support for elfutils

When elfutils was compiled before, libglib2 will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/gresource | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libgio-2.0.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libgobject-2.0.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libgmodule-2.0.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libglib-2.0.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libelf.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libintl.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>