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>
---
diff --git a/scripts/kconfig/lxdialog/Makefile b/scripts/kconfig/lxdialog/Makefile
index fae3e29..bbf4887 100644
--- a/scripts/kconfig/lxdialog/Makefile
+++ b/scripts/kconfig/lxdialog/Makefile
@@ -2,8 +2,11 @@
 #
 
 check-lxdialog  := $(srctree)/$(src)/check-lxdialog.sh
-HOST_EXTRACFLAGS:= $(shell $(CONFIG_SHELL) $(check-lxdialog) -ccflags)
-HOST_LOADLIBES  := $(shell $(CONFIG_SHELL) $(check-lxdialog) -ldflags $(HOSTCC))
+
+# Use reursively expanded variables so we do not call gcc unless
+# we really need to do so. (Do not call gcc as part of make mrproper)
+HOST_EXTRACFLAGS = $(shell $(CONFIG_SHELL) $(check-lxdialog) -ccflags)
+HOST_LOADLIBES   = $(shell $(CONFIG_SHELL) $(check-lxdialog) -ldflags $(HOSTCC))
  
 HOST_EXTRACFLAGS += -DLOCALE