Boost gets tag "compiled-by-unknown_host-compiler"

[psyhtest]

I have noticed that when Boost is installed as a Caffe dependency (and possibly standalone), in addition to compiled-by-gcc and and compiled-by-gcc-<version tags, it gets a weird compiled-by-unknown_host-compiler tag:

$ ck show env --tags=boost,lib
Env UID:         Target OS: Bits: Name:         Version:             Tags:

49673bc810e4d8e9   linux-64    64 Boost library 1.67.0-min-for-caffe 64bits,boost,channel-stable,compiled-by-gcc,compiled-by-gcc-8.1.0,compiled-by-unknown_host-compiler,host-os-linux-64,lib,min-for-caffe,target-os-linux-64,v1,v1.67,v1.67.0,v1.67.0-min-for-caffe,v1.67.0.0,v1.67.0.0.0,v1.67.0.0.0.0
002ceccabd454515   linux-64    64 Boost library 1.67.0-min-for-caffe 64bits,boost,channel-stable,compiled-by-llvm,compiled-by-llvm-5.0.0-3~16.04.1,compiled-by-unknown_host-compiler,host-os-linux-64,lib,min-for-caffe,target-os-linux-64,v1,v1.67,v1.67.0,v1.67.0-min-for-caffe,v1.67.0.0,v1.67.0.0.0,v1.67.0.0.0.0
c0aefcb48a2b39f7   linux-64    64 Boost library 1.67.0-min-for-caffe 64bits,boost,channel-stable,compiled-by-gcc,compiled-by-gcc-7.3.0,compiled-by-unknown_host-compiler,host-os-linux-64,lib,min-for-caffe,target-os-linux-64,v1,v1.67,v1.67.0,v1.67.0-min-for-caffe,v1.67.0.0,v1.67.0.0.0,v1.67.0.0.0.0
90bbc584d0260b58   linux-64    64 Boost library 1.66.0-min-for-caffe 64bits,boost,channel-stable,compiled-by-gcc,compiled-by-gcc-6.4.0,compiled-by-unknown_host-compiler,host-os-linux-64,lib,min-for-caffe,target-os-linux-64,v1,v1.66,v1.66.0,v1.66.0-min-for-caffe,v1.66.0.0,v1.66.0.0.0,v1.66.0.0.0.0
508a59286c69b492   linux-64    64 Boost library 1.66.0               64bits,boost,channel-stable,compiled-by-gcc,compiled-by-gcc-6.4.0,compiled-by-unknown_host-compiler,host-os-linux-64,lib,target-os-linux-64,v1,v1.66,v1.66.0

[psyhtest]

The same tag gets added when compiling Boost standalone:

$ ck install package:lib-boost-1.64.0-min-for-caffe
$ ck show env --tags=lib,boost,v1.64
Env UID:         Target OS: Bits: Name:         Version:             Tags:

822b4e5283a69e6f   linux-64    64 Boost library 1.64.0-min-for-caffe 64bits,boost,channel-stable,compiled-by-llvm,compiled-by-llvm-4.0.0-1ubuntu1~16.04.2,compiled-by-unknown_host-compiler,host-os-linux-64,lib,min-for-caffe,target-os-linux-64,v1,v1.64,v1.64.0,v1.64.0-min-for-caffe,v1.64.0.0,v1.64.0.0.0,v1.64.0.0.0.0

Why is that?