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/uclibc/90_all_103-uclibc-conf-noupstream.patch

diff -durN gcc-4.3.1.orig/gcc/config.gcc gcc-4.3.1/gcc/config.gcc

--- gcc-4.3.1.orig/gcc/config.gcc	2008-06-10 14:58:09.000000000 +0200

+++ gcc-4.3.1/gcc/config.gcc	2008-06-10 14:58:36.000000000 +0200

@@ -2149,7 +2149,7 @@

         ;;

 sh-*-elf* | sh[12346l]*-*-elf* | sh*-*-kaos* | \

 sh-*-symbianelf* | sh[12346l]*-*-symbianelf* | \

-  sh-*-linux* | sh[346lbe]*-*-linux* | \

+  sh*-*-linux* | sh[346lbe]*-*-linux* | \

   sh-*-netbsdelf* | shl*-*-netbsdelf* | sh5-*-netbsd* | sh5l*-*-netbsd* | \

    sh64-*-netbsd* | sh64l*-*-netbsd*)

 	tmake_file="${tmake_file} sh/t-sh sh/t-elf"