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 -durN glibc-cvs-2.9.orig/misc/syslog.c glibc-cvs-2.9/misc/syslog.c

 --- glibc-cvs-2.9.orig/misc/syslog.c	2009-06-01 10:16:50.000000000 +0200

 +++ glibc-cvs-2.9/misc/syslog.c	2009-06-01 10:17:20.000000000 +0200

 @@ -152,7 +152,7 @@

  #define	INTERNALLOG	LOG_ERR|LOG_CONS|LOG_PERROR|LOG_PID

  	/* Check for invalid bits. */

  	if (pri & ~(LOG_PRIMASK|LOG_FACMASK)) {

 -		syslog(INTERNALLOG,

 +		__syslog(INTERNALLOG,

  		    "syslog: unknown facility/priority: %x", pri);

  		pri &= LOG_PRIMASK|LOG_FACMASK;

  	}