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]);