mk4build: change how the _FOR_BUILD variables are passed to sub-configure
mk4build: change how the _FOR_BUILD variables are passed to sub-configure

The logic that eval's for *_FOR_BUILD variables to set them to their
equivalent without _FOR_BUILD before calling the sub-configure for the
host machine doesn't work: the variables are not exported, and
therefore they are not seen by the ./configure script, which runs as a
separate process.

So instead, keep things simple and pass them explicitly in the
environment of the ./configure script.

This patch was written in the context of the Buildroot build system,
which does cross-compilation, through which we discovered that the
*_FOR_BUILD variables are not properly passed down to the
sub-configure.

Signed-off-by: Thomas Petazzoni <a5d87d2bd7c424ee5cbdb4e978635bf653d5a005@bootlin.com>

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