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: ../4.3.2/140-netbsd-symbolic.patch

-= BEGIN original header =-

Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/11_all_gcc-netbsd-symbolic.patch

http://bugs.gentoo.org/122698

-= END original header =-

diff -durN gcc-4.3.3.orig/gcc/config/netbsd-elf.h gcc-4.3.3/gcc/config/netbsd-elf.h

--- gcc-4.3.3.orig/gcc/config/netbsd-elf.h	2007-09-03 18:14:04.000000000 +0200

+++ gcc-4.3.3/gcc/config/netbsd-elf.h	2009-01-27 22:19:18.000000000 +0100

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

 #define NETBSD_LINK_SPEC_ELF \

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

    %{shared:-shared} \

+   %{symbolic:-Bsymbolic} \

    %{!shared: \

      -dc -dp \

      %{!nostdlib: \