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 -urN uClibc-dist/libc/sysdeps/linux/common/bits/uClibc_touplow.h uClibc/libc/sysdeps/linux/common/bits/uClibc_touplow.h

 --- uClibc-dist/libc/sysdeps/linux/common/bits/uClibc_touplow.h   Fri Jun  3 13:30:25 2005

 +++ uClibc/libc/sysdeps/linux/common/bits/uClibc_touplow.h   Fri Jun  3 13:30:39 2005

 @@ -34,11 +34,11 @@

  /* glibc uses the equivalent of - typedef __int32_t __ctype_touplow_t; */

 -typedef __uint16_t __ctype_mask_t;

 +typedef __uint32_t __ctype_mask_t;

  #ifdef __UCLIBC_HAS_CTYPE_SIGNED__

 -typedef __int16_t __ctype_touplow_t;

 +typedef __int32_t __ctype_touplow_t;

  #define __UCLIBC_CTYPE_B_TBL_OFFSET       128

  #define __UCLIBC_CTYPE_TO_TBL_OFFSET      128