Mesa ebuild doesn't force installation of libdrm with abi_x86_32:
checking for the pthreads library -lpthreads... no
checking whether pthreads work without any flags... no
checking whether pthreads work with -Kthread... no
checking whether pthreads work with -kthread... no
checking for the pthreads library -llthread... no
checking whether pthreads work with -pthread... yes
checking for joinable pthread attribute... PTHREAD_CREATE_JOINABLE
checking if more special flags are required for pthreads... no
checking for PTHREAD_PRIO_INHERIT... no
checking for LIBDRM... no
checking for GLPROTO... yes
configure: error: Direct rendering requires libdrm >= 2.4.24
Mesa ebuild doesn't force installation of libdrm with abi_x86_32: