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>

 --- glibc-2.6.1/sysdeps/unix/sysv/linux/kernel-features.h	2006-07-29 07:43:27.000000000 +0300

 +++ glibc-2.6.1/sysdeps/unix/sysv/linux/kernel-features.h.new	2008-07-26 08:45:06.000000000 +0300

 @@ -445,7 +445,7 @@

  /* The *at syscalls were introduced just after 2.6.16-rc1.  Due to the way the

     kernel versions are advertised we can only rely on 2.6.17 to have

     the code.  */

 -#if __LINUX_KERNEL_VERSION >= 0x020611

 +#if __LINUX_KERNEL_VERSION >= 0x020611 && !defined __alpha__

  # define __ASSUME_ATFCTS	1

  #endif