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>

 Original patch from http://www.nabble.com/arm-linux-compilation-failure-and-possible-fix-td19229074.html

 diff -durN glibc-2.9.orig/ports/sysdeps/unix/sysv/linux/arm/sysdep.h glibc-2.9/ports/sysdeps/unix/sysv/linux/arm/sysdep.h

 --- glibc-2.9.orig/ports/sysdeps/unix/sysv/linux/arm/sysdep.h 25 Nov 2008 16:37:26 -0000 1.33

 +++ glibc-2.9/ports/sysdeps/unix/sysv/linux/arm/sysdep.h 27 Jan 2009 15:35:38 -0000

 @@ -28,6 +28,8 @@

  /* Defines RTLD_PRIVATE_ERRNO and USE_DL_SYSINFO.  */

  #include <dl-sysdep.h>

 +#include <tls.h>

 +

  /* For Linux we can use the system call table in the header file

 	/usr/include/asm/unistd.h

     of the kernel.  But these symbols do not follow the SYS_* syntax