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>

 many ports hit this warning:

 ../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

 ../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

 snipped from suse

 diff -durN glibc-2.12.1.orig/sysdeps/unix/sysv/linux/check_native.c glibc-2.12.1/sysdeps/unix/sysv/linux/check_native.c

 --- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/check_native.c	2007-11-24 04:12:17.000000000 +0100

 +++ glibc-2.12.1/sysdeps/unix/sysv/linux/check_native.c	2009-11-13 00:50:11.000000000 +0100

 @@ -23,6 +23,7 @@

  #include <stddef.h>

  #include <stdint.h>

  #include <stdlib.h>

 +#include <string.h>

  #include <time.h>

  #include <unistd.h>

  #include <net/if.h>