Changing the gl options to qemu doesn't result in a correctly rebuilt
binary, the GL linkage can persist from a build where it was enabled
to one where it was not.
As well as clearly being incorrect and non-reproducible, this caused
some mystery failures on the autobuilder.
Cleaning ${B} at do_configure time avoids this. Most recipes
(e.g. autotools derived ones) already clean ${B} as appropriate and
avoid this issue.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
