Since commit d3f86c9cc3 tools/libelf
installs a pkgconfig file, which in turn is picked up during the glib2
host build. On the build bots the same occurs, although it so happens
that (sometimes?) libelf.h is not found and the host build fails. This
may be a timing issue specific to the build bots, as the header is in
fact installed by tools/libelf and I can't reproduce this here on my own
computer.

In any case, libelf (if detected) is only used by gresource. gresource
can be used without it. gresource is not used by OpenWrt's build system
(also not in openwrt/packages). So adding "--disable-libelf" to the host
configure args is not detrimental in any way and works around the issue
described above.

Resolves #9927

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>