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 -u binutils-2.18-orig/configure binutils-2.18/configure

 --- binutils-2.18-orig/configure	2007-08-06 16:29:40.000000000 -0400

 +++ binutils-2.18/configure	2007-09-27 22:41:51.000000000 -0400

 @@ -6125,10 +6125,10 @@

    *" texinfo "*) MAKEINFO='$$r/$(BUILD_SUBDIR)/texinfo/makeinfo/makeinfo' ;;

    *)

 -    # For an installed makeinfo, we require it to be from texinfo 4.4 or

 +    # For an installed makeinfo, we require it to be from texinfo 4.6 or

      # higher, else we use the "missing" dummy.

      if ${MAKEINFO} --version \

 -       | egrep 'texinfo[^0-9]*([1-3][0-9]|4\.[4-9]|[5-9])' >/dev/null 2>&1; then

 +       | egrep 'texinfo[^0-9]*(4\.([6-9]|[1-9][0-9])|[5-9]|[1-9][0-9])' >/dev/null 2>&1; then

        :

      else

        MAKEINFO="$MISSING makeinfo"