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 gcc-4.4.0.orig/include/libiberty.h gcc-4.4.0/include/libiberty.h

 --- gcc-4.4.0.orig/include/libiberty.h	2008-06-24 03:42:31.000000000 +0200

 +++ gcc-4.4.0/include/libiberty.h	2009-05-27 21:38:24.000000000 +0200

 @@ -595,8 +595,11 @@

  /* Like sprintf but provides a pointer to malloc'd storage, which must

     be freed by the caller.  */

 +/* asprintf may be declared as a macro by glibc with __USE_FORTIFY_LEVEL.  */

 +#ifndef asprintf

  extern int asprintf (char **, const char *, ...) ATTRIBUTE_PRINTF_2;

  #endif

 +#endif

  #if !HAVE_DECL_VASPRINTF

  /* Like vsprintf but provides a pointer to malloc'd storage, which