This is a bug against Gentoo, not upstream, as Gentoo ignores upstream's buildsystem (and upstream seems quite opaque regardless). See to-be-attached build.log.
Created attachment 243141 [details] /tmp/libhash-1.0.3-build.log build log with the warning.
Created attachment 243143 [details] /tmp/libhash-1.0.3-emerge--info.txt emerge --info
Created attachment 243145 [details] libhash-1.0.3.ebuild-soname.patch This patch adds -soname to the libhash-1.0.3 ebuild. It does not fix libhash-1.0.2.ebuild, which should be removed as libhash-1.0.2 doesn't even respect LDFLAGS.
Fixed in -r1.