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>

--- glibc-2.5/sysdeps/unix/sysv/linux/alpha/sigsuspend.S.orig	2006-10-22 22:17:01.000000000 +0200

+++ glibc-2.5/sysdeps/unix/sysv/linux/alpha/sigsuspend.S	2006-10-22 22:18:05.000000000 +0200

@@ -31,3 +31,21 @@

 libc_hidden_def (__sigsuspend)

 weak_alias (__sigsuspend, sigsuspend)

 strong_alias (__sigsuspend, __libc_sigsuspend)

+

+#ifndef NO_CANCELLATION

+	.globl __sigsuspend_nocancel;

+	.align 4;

+	.type __sigsuspend_nocancel, @function;

+	.usepv __sigsuspend_nocancel, std;

+	cfi_startproc;

+__LABEL(__sigsuspend_nocancel)

+	ldgp    gp, 0(pv);

+	PSEUDO_PROF;

+	PSEUDO_PREPARE_ARGS

+	lda     v0, SYS_ify(sigsuspend);

+	call_pal PAL_callsys;

+	bne     a3, SYSCALL_ERROR_LABEL;

+	ret;

+	cfi_endproc;

+	.size __sigsuspend_nocancel, .-__sigsuspend_nocancel

+#endif