I have had this error a couple of times, and each time it took a while to figure out the problem.
Guile, gnutls or some other package fails to find libunistring during the configure stage of emerge.
Compile libunistring locally (belt and suspenders version):
FEATURES="-ditcc -distcc-pump" MAKEOPTS="-j1" emerge -j1 libunistring
Generated on 2018-05-03 01:14:21.916040