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>

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/66_all_gcc43-pr25343.patch

 http://gcc.gnu.org/PR25343

 sniped from Debian

 2008-04-27  Roman Zippel <zippel@linux-m68k.org>

 	* config/host-linux.c (TRY_EMPTY_VM_SPACE): Define for __mc68000__.

 diff -durN gcc-4.3.1.orig/gcc/config/host-linux.c gcc-4.3.1/gcc/config/host-linux.c

 --- gcc-4.3.1.orig/gcc/config/host-linux.c	2007-08-02 12:49:31.000000000 +0200

 +++ gcc-4.3.1/gcc/config/host-linux.c	2008-06-10 14:58:28.000000000 +0200

 @@ -84,6 +84,8 @@

  # define TRY_EMPTY_VM_SPACE	0x8000000000

  #elif defined(__sparc__)

  # define TRY_EMPTY_VM_SPACE	0x60000000

 +#elif defined(__mc68000__)

 +# define TRY_EMPTY_VM_SPACE	0x40000000

  #else

  # define TRY_EMPTY_VM_SPACE	0

  #endif