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/binutils/2.19/78_all_use-relro.patch

 diff -durN binutils-2.19.orig/ld/ldmain.c binutils-2.19/ld/ldmain.c

 --- binutils-2.19.orig/ld/ldmain.c	2008-11-23 16:59:24.000000000 +0100

 +++ binutils-2.19/ld/ldmain.c	2008-11-23 16:59:45.000000000 +0100

 @@ -264,6 +264,7 @@

    link_info.keep_memory = TRUE;

    link_info.new_dtags = TRUE;

    link_info.combreloc = TRUE;

 +  link_info.relro = TRUE;

    link_info.strip_discarded = TRUE;

    link_info.emit_hash = TRUE;

  #ifndef __mips__