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-2.7/sysdeps/unix/sysv/linux/kernel-features.h	2007-08-14 06:21:09.000000000 +0300

+++ glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h.new	2008-07-23 07:05:20.000000000 +0300

@@ -454,8 +454,7 @@

    and on Alpha just after 2.6.22-rc1.  */

 #if __LINUX_KERNEL_VERSION >= 0x020611 \

     && ((!defined __sh__ && !defined __alpha__) \

-	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__) \

-	|| (__LINUX_KERNEL_VERSION >= 0x020617 && defined __alpha__))

+	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__))

 # define __ASSUME_ATFCTS	1

 #endif