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 -durN gcc-3.4.6.orig/gcc/config/sh/sh.h gcc-3.4.6/gcc/config/sh/sh.h

 --- gcc-3.4.6.orig/gcc/config/sh/sh.h	2006-01-12 02:20:32.000000000 +0100

 +++ gcc-3.4.6/gcc/config/sh/sh.h	2007-08-15 23:03:09.000000000 +0200

 @@ -422,6 +422,10 @@

  do {									\

    if (LEVEL)								\

      flag_omit_frame_pointer = -1;					\

 +  if (LEVEL <= 2)							\

 +    {									\

 +      flag_reorder_blocks = 0;						\

 +    }									\

    if (SIZE)								\

      target_flags |= SPACE_BIT;						\

    if (TARGET_SHMEDIA && LEVEL > 1)					\