CMAKE_{AR,STRIP,LINKER,RANLIB,OBJCOPY} are set incorrectly when overridden

I've tried this with CMake 3.5 and 3.11, on GNU/Linux systems.

If I have a simple CMakeLists.txt file:

$ cd /tmp/obj

$ cat ../src/CMakeLists.txt
PROJECT(foobar)
CMAKE_MINIMUM_REQUIRED(VERSION 3.0)

Then I run cmake while overriding any of the variables in the subject:

$ type -a mklib
/tools/bin/mklib

$ mklib
mklib version 1.0

$ cmake CMAKE_AR=mklib ../src

Then cmake does completely the wrong thing and assumes that this tool exists in the local directory rather than on the PATH where it really exists:

$ grep mklib CMakeCache.txt
CMAKE_AR:FILEPATH=/tmp/obj/mklib
//ADVANCED property for variable: CMAKE_AR
CMAKE_AR-ADVANCED:INTERNAL=1

This obviously causes my builds to fail since that binary doesn't exist at that path.

The same behavior happens for the CMAKE_LINKER, CMAKE_RANLIB, CMAKE_STRIP, and CMAKE_OBJCOPY variable assignments.

Oddly, the CMAKE_C_COMPILER and CMAKE_CXX_COMPILER variables do work properly (that is, if I override them with a simple command name that command is looked up in the PATH and the correct value is substituted rather than assuming the local directory).