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-4.3.2.orig/libgcc/config/sh/t-linux gcc-4.3.2/libgcc/config/sh/t-linux

 --- gcc-4.3.2.orig/libgcc/config/sh/t-linux	2009-03-07 11:42:38.000000000 +0100

 +++ gcc-4.3.2/libgcc/config/sh/t-linux	2009-03-07 11:03:39.000000000 +0100

 @@ -15,7 +15,7 @@

  	-Wl,--soname=@shlib_base_name@.so.1 \

  	-Wl,--version-script=@shlib_map_file@ \

  	-o @multilib_dir@/@shlib_base_name@.so.1.tmp @multilib_flags@ \

 -	@shlib_objs@ -lc && \

 +	@shlib_objs@ && \

  	rm -f @multilib_dir@/@shlib_base_name@.so && \

  	if [ -f @multilib_dir@/@shlib_base_name@.so.1 ]; then \

  	  mv -f @multilib_dir@/@shlib_base_name@.so.1 \