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>

 diff -dur strace-4.5.15.orig/net.c strace-4.5.15/net.c

 --- strace-4.5.15.orig/net.c	2006-12-13 20:57:23.000000000 +0100

 +++ strace-4.5.15/net.c	2007-07-14 19:29:36.000000000 +0200

 @@ -800,8 +800,12 @@

  #ifdef SOL_PACKET

  static const struct xlat sockpacketoptions[] = {

 +#if defined(PACKET_ADD_MEMBERSHIP)

  	{ PACKET_ADD_MEMBERSHIP,	"PACKET_ADD_MEMBERSHIP"	},

 +#endif

 +#if defined(PACKET_DROP_MEMBERSHIP)

  	{ PACKET_DROP_MEMBERSHIP,	"PACKET_DROP_MEMBERSHIP"},

 +#endif

  #if defined(PACKET_RECV_OUTPUT)

  	{ PACKET_RECV_OUTPUT,		"PACKET_RECV_OUTPUT"	},

  #endif