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-4.4.0.orig/gcc/config.gcc gcc-4.4.0/gcc/config.gcc

 --- gcc-4.4.0.orig/gcc/config.gcc	2009-04-17 13:58:41.000000000 +0200

 +++ gcc-4.4.0/gcc/config.gcc	2009-05-27 21:38:31.000000000 +0200

 @@ -2121,7 +2121,7 @@

  	if test x${sh_multilibs} = x ; then

  		case ${target} in

  		sh64-superh-linux* | \

 -		sh[1234]*)	sh_multilibs=${sh_cpu_target} ;;

 +		sh[1234]*)	sh_multilibs=`cd ${srcdir}/config/sh ; echo t-mlib-sh[1-4]* | sed 's:t-mlib-sh:,m:g;s: ::g'` ;;

  		sh64* | sh5*)	sh_multilibs=m5-32media,m5-32media-nofpu,m5-compact,m5-compact-nofpu,m5-64media,m5-64media-nofpu ;;

  		sh-superh-*)	sh_multilibs=m4,m4-single,m4-single-only,m4-nofpu ;;

  		sh*-*-linux*)	sh_multilibs=m1,m3e,m4 ;;