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 -ru ltrace-0.5.3.orig/sysdeps/linux-gnu/Makefile ltrace-0.5.3/sysdeps/linux-gnu/Makefile

--- ltrace-0.5.3.orig/sysdeps/linux-gnu/Makefile	2009-07-25 17:13:02.000000000 +0200

+++ ltrace-0.5.3/sysdeps/linux-gnu/Makefile	2010-01-01 22:20:45.000000000 +0100

@@ -2,6 +2,7 @@

 				-e s/i.86/i386/ \

 				-e s/sun4u/sparc64/ \

 				-e s/sparc64/sparc/ \

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

 				-e s/arm.*/arm/ \

 				-e s/sa110/arm/ \

 				-e s/ppc64/ppc/ \