lipcap build also requires lib=lib in the LIBCAP_BUILD_CMDS or it pulls the lib from the host os. I would suggest removing the lib=lib from the libcap.mk and adding a patch to fix Make.Rules file instead.
Thanks for the report! Since it seems you have investigated the issue and found a solution, do you think you could send a patch to the Buildroot mailing list to fix it ? Thanks!
Sure, I'll educate myself on how to do that.
Fixed on master, thanks: https://git.buildroot.org/buildroot/commit/?id=07f8ea39139299c83777a338a30cd480633d5706