For rtlsdr use, the depending should be rtlsdr? ( >=net-wireless/rtl-sdr-0.5.4:= ) instead of 0.5.3. The error description: https://lists.osmocom.org/pipermail/osmocom-sdr/2017-June/001595.html I verified it for 0.1.4_p20180626-r2 version.
fixed in r4 and r5. thanks