kconfig: fix /dev/null breakage

While running "make menuconfig" and "make mrproper"
some people experienced that /dev/null suddenly changed
permissions or suddenly became a regular file.
The main reason was that /dev/null was used as output
to gcc in the check-lxdialog.sh script and gcc did
some strange things with the output file; in this
case /dev/null when it errorred out.

Following patch implements a suggestion
from Bryan O'Sullivan <bos@serpentine.com> to
use gcc -print-file-name=libxxx.so.

Also the Makefile is adjusted to not resolve value of
HOST_EXTRACFLAGS and HOST_LOADLIBES until they are actually used.
This prevents us from calling gcc when running make *clean/mrproper

Thanks to Eyal Lebedinsky <eyal@eyal.emu.id.au> and
Jean Delvare <khali@linux-fr.org> for the first error reports.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
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