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>

 # This file declares functions to install the kernel headers for mingw

 # Copyright 2009 Bart vdr. Meulen

 # Licensed under the GPL v2. See COPYING in the root of this package

 CT_DoKernelTupleValues() {

     CT_TARGET_KERNEL="mingw32"

     CT_TARGET_SYS=

 }

 do_kernel_get() {

     CT_GetFile "w32api-${CT_W32API_VERSION}-mingw32-src" \

         http://downloads.sourceforge.net/sourceforge/mingw

 }

 do_kernel_extract() {

     CT_Extract "w32api-${CT_W32API_VERSION}-mingw32-src"

 }

 do_kernel_headers() {

     CT_DoStep INFO "Installing kernel headers"

     mkdir -p "${CT_HEADERS_DIR}"

     cp -r ${CT_SRC_DIR}/w32api-${CT_W32API_VERSION}-mingw32/include/* \

           ${CT_HEADERS_DIR}

     CT_EndStep

 }