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>

--- libc/nptl/sysdeps/unix/sysv/linux/alpha/sysdep-cancel.h    2006/10/29 21:48:06     1.6

+++ libc/nptl/sysdeps/unix/sysv/linux/alpha/sysdep-cancel.h    2007/03/13 21:28:03     1.7

@@ -55,6 +55,7 @@

 	bne	a3, SYSCALL_ERROR_LABEL;			\

 __LABEL($pseudo_ret)						\

 	.subsection 2;						\

+	cfi_startproc;						\

 __LABEL($pseudo_cancel)						\

 	subq	sp, 64, sp;					\

 	cfi_def_cfa_offset(64);					\

@@ -90,12 +91,13 @@

 	cfi_def_cfa_offset(0);					\

 __LABEL($syscall_error)						\

 	SYSCALL_ERROR_HANDLER;					\

+	cfi_endproc;						\

 	.previous

 # undef PSEUDO_END

 # define PSEUDO_END(sym)					\

-	.subsection 2;						\

 	cfi_endproc;						\

+	.subsection 2;						\

 	.size sym, .-sym

 # define SAVE_ARGS_0	/* Nothing.  */