In Gentoo and ChromeOS we want to allow pure LLVM builds without
using GNU tools, so we block any unwanted mixed GNU/LLVM usages
(GNU tools are still kept around in our chroots for projects
like glibc which cannot yet be built otherwise).

The default ${cross_prefix}${ranlib_default} points to GNU and
fails, so move the test a bit later - after the defaults are
set and the proper values get overriden - such that ffmpeg
configure calls the llvm-ranlib we desire. [1]

[1] https://gitweb.gentoo.org/repo/gentoo.git/tree/media-video/ffmpeg/ffmpeg-4.4.1-r1.ebuild?id=7a34377e3277a6a0e2eedd40e90452a44c55f1e6#n477Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Martin Storsjö <martin@martin.st>