The package app-emulation/qemu-guest-agent-1.6.0, which provides the guest tools under KVM virtualization, fails to build with the following message:

ar: two different operation options specified
Makefile:234: recipe for target 'libfdt/libfdt.a' failed

I've been able to compile the package exporting ARFLAGS:

ARFLAGS="rv" emerge qemu-guest-agent

This bug seems to be related with use of >=sys-devel/make-4.0 and needs further investigation. As starting point the following thread must be read:

http://lists.gnu.org/archive/html/bug-make/2013-10/msg00151.html