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 --git a/mpz/remove.c b/mpz/remove.c

 --- a/mpz/remove.c

 +++ b/mpz/remove.c

 @@ -23,7 +23,7 @@

  unsigned long int

  mpz_remove (mpz_ptr dest, mpz_srcptr src, mpz_srcptr f)

  {

 -  mpz_t fpow[40];		/* inexhaustible...until year 2020 or so */

 +  mpz_t fpow[GMP_LIMB_BITS];		/* Really MP_SIZE_T_BITS */

    mpz_t x, rem;

    unsigned long int pwr;

    int p;

 @@ -69,7 +69,7 @@

        mpz_set (dest, x);

      }

 -  pwr = (1 << p) - 1;

 +  pwr = (1L << p) - 1;

    mpz_clear (fpow[p]);

 diff --git a/mpz/remove.c b/mpz/remove.c

 --- a/mpz/remove.c

 +++ b/mpz/remove.c

 @@ -80,7 +80,7 @@

        mpz_tdiv_qr (x, rem, dest, fpow[p]);

        if (SIZ (rem) == 0)

  	{

 -	  pwr += 1 << p;

 +	  pwr += 1L << p;

  	  mpz_set (dest, x);

  	}

        mpz_clear (fpow[p]);