cross compiling libc++ on OSX (overriding ranlib and ar selections)

What is the proper way to override ranlib and ar with the cmake system in libc++ so that the proper versions can be specified for cross compiling? I have a cross-compiler for embedded linux set up on OSX. I just spent two days figuring out why I could not get a static build of libc++ to link properly and it was because cmake was not using the elf versions from my cross-toolchain. nothing I tried would get the cmake scripts to override its decisions with mine.