Originally Posted by DanL
Only the version of libdrm is tested; libdrm_radeon is just checked for existence.
PKG_CHECK_MODULES([LIBDRM_RADEON], [libdrm_radeon libdrm >= $LIBDRM_RADEON_REQUIRED], HAVE_LIBDRM_RADEON=yes, HAVE_LIBDRM_RADEON=no)
Edit: to clarify: mesa itself works with older libdrm (2.4.15); however in order to use the new API it requires that libdrm_radeon is installed and libdrm is >= 2.4.17.
The message above may be confusing since it refers to libdrm_radeon but in reality it checks two conditions.