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>

 Signed-off-by: dank@kegel.com

 Raising the size of the hash table is a noticable win when linking 

 at least one large app.  

 There was a patch,

 http://sources.redhat.com/ml/binutils/2004-06/msg00165.html

 to do this dynamically.  No idea why that didn't make it in.

 So this tiny change to raise the default size is just

 a stopgap for now.

 --- binutils-2.16.1/bfd/hash.c.old	2006-03-02 23:01:39.000000000 -0800

 +++ binutils-2.16.1/bfd/hash.c	2006-03-02 23:01:58.000000000 -0800

 @@ -300,7 +300,7 @@

  */

  /* The default number of entries to use when creating a hash table.  */

 -#define DEFAULT_SIZE 4051

 +#define DEFAULT_SIZE 32749

  static size_t bfd_default_hash_table_size = DEFAULT_SIZE;

  /* Create a new hash table, given a number of entries.  */