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>

# dmalloc debug facility

## depends on ! BACKEND

##

## help dmalloc helps tracking memory allocation mis-usage

## help such as double-free, use-after-free...

choice

    bool

    prompt "dmalloc version"

# Don't remove next line

# CT_INSERT_VERSION_BELOW

config DMALLOC_V_5_5_2

    bool

    prompt "5.5.2"

endchoice

config DMALLOC_VERSION

    string

# Don't remove next line

# CT_INSERT_VERSION_STRING_BELOW

    default "5.5.2" if DMALLOC_V_5_5_2