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>

 http://sourceware.org/ml/binutils/2007-07/msg00401.html

 http://sourceware.org/bugzilla/show_bug.cgi?id=4970

 diff -durN binutils-2.20.1.orig/configure binutils-2.20.1/configure

 --- binutils-2.20.1.orig/configure	2010-03-03 14:59:02.000000000 +0100

 +++ binutils-2.20.1/configure	2010-08-17 19:32:05.000000000 +0200

 @@ -6466,6 +6466,7 @@

    *-*-mingw* | *-*-cygwin ) RPATH_ENVVAR=PATH ;;

    *) RPATH_ENVVAR=LD_LIBRARY_PATH ;;

  esac

 +RPATH_ENVVAR="cant_touch_this_nah_nah_nah"

  # On systems where the dynamic library environment variable is PATH,

  # gcc/ will put dynamic libraries into a subdirectory to avoid adding