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 glibc-2.12.1.orig/sysdeps/unix/sysv/linux/kernel-features.h glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h

--- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/kernel-features.h	2009-05-09 08:54:20.000000000 +0200

+++ glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h	2009-11-13 00:50:45.000000000 +0100

@@ -386,6 +386,11 @@

 # define __ASSUME_GETDENTS32_D_TYPE	1

 #endif

+/* Starting with version 2.6.4, alpha stat64 syscalls are available.  */

+#if __LINUX_KERNEL_VERSION >= 0x020604 && defined __alpha__

+# define __ASSUME_STAT64_SYSCALL	1

+#endif

+

 /* Starting with version 2.5.3, the initial location returned by `brk'

    after exec is always rounded up to the next page.  */

 #if __LINUX_KERNEL_VERSION >= 132355