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>

--- gcc-4.1.2.orig/gcc/configure.ac	2006-11-13 23:09:55.000000000 +0100

+++ gcc-4.1.2/gcc/configure.ac	2007-03-04 23:34:53.000000000 +0100

@@ -2435,7 +2435,7 @@

 	tls_first_minor=14

 	tls_as_opt="-m64 -Aesame --fatal-warnings"

 	;;

-  sh-*-* | sh[34]-*-*)

+  sh-*-* | sh[34]*-*-*)

     conftest_s='

 	.section ".tdata","awT",@progbits

 foo:	.long	25

--- gcc-4.1.2.orig/gcc/configure	2006-11-13 23:09:55.000000000 +0100

+++ gcc-4.1.2/gcc/configure	2007-03-04 23:34:53.000000000 +0100

@@ -14762,7 +14762,7 @@

 	tls_first_minor=14

 	tls_as_opt="-m64 -Aesame --fatal-warnings"

 	;;

-  sh-*-* | sh[34]-*-*)

+  sh-*-* | sh[34]*-*-*)

     conftest_s='

 	.section ".tdata","awT",@progbits

 foo:	.long	25