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>

--- a/extra/scripts/unifdef.c

+++ b/extra/scripts/unifdef.c

@@ -206,7 +206,7 @@

 static void             error(const char *);

 static int              findsym(const char *);

 static void             flushline(bool);

-static Linetype         getline(void);

+static Linetype         get_line(void);

 static Linetype         ifeval(const char **);

 static void             ignoreoff(void);

 static void             ignoreon(void);

@@ -512,7 +512,7 @@

 	for (;;) {

 		linenum++;

-		lineval = getline();

+		lineval = get_line();

 		trans_table[ifstate[depth]][lineval]();

 		debug("process %s -> %s depth %d",

 		    linetype_name[lineval],

@@ -526,7 +526,7 @@

  * help from skipcomment().

  */

 static Linetype

-getline(void)

+get_line(void)

 {

 	const char *cp;

 	int cursym;