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/gentoo/11_all_gcc-netbsd-symbolic.patch

 http://bugs.gentoo.org/122698

 diff -durN gcc-4.3.1.orig/gcc/config/netbsd-elf.h gcc-4.3.1/gcc/config/netbsd-elf.h

 --- gcc-4.3.1.orig/gcc/config/netbsd-elf.h	2007-09-03 18:14:04.000000000 +0200

 +++ gcc-4.3.1/gcc/config/netbsd-elf.h	2008-06-10 14:57:48.000000000 +0200

 @@ -82,6 +82,7 @@

  #define NETBSD_LINK_SPEC_ELF \

    "%{assert*} %{R*} %{rpath*} \

     %{shared:-shared} \

 +   %{symbolic:-Bsymbolic} \

     %{!shared: \

       -dc -dp \

       %{!nostdlib: \