configure: add possibility to set arbitrary variable in check_for

If check_for is able to find the required prog/inc/lib, allow it to
set an arbitrary variable to 'y'. This variable is then pushed down
to the kconfig definition.

For example:
has_or_abort prog=foobar kconfig=has_foobar

If foobar is available, it yields a kconfig variable defaulting to y:
config CONFIGURE_has_foobar
bool
default y

If foobar is missing, it yields a kconfig variable defaulting to n:
config CONFIGURE_has_foobar
bool

Thus it is possible to depends on that variabel to show/hide options:
config SOME_FEATURE
bool
prompt "Some feature"
depends on CONFIGURE_has_foobar

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

diff -durN ltrace-0.4.orig/Makefile.in ltrace-0.4/Makefile.in

--- ltrace-0.4.orig/Makefile.in	2008-10-26 15:34:21.000000000 +0100

+++ ltrace-0.4/Makefile.in	2008-10-26 15:37:40.000000000 +0100

@@ -5,6 +5,7 @@

 OS		:= @HOST_OS@

 ARCH		:= $(shell echo "@HOST_ARCH@" |sed -e s/i.86/i386/      \

 						   -e s/sun4u/sparc64/  \

+						   -e s/alpha.*/alpha/  \

 						   -e s/arm.*/arm/      \

 						   -e s/sa110/arm/      \

 						   -e s/ppc64/ppc/      \