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 -dur strace-4.5.15.orig/configure.ac strace-4.5.15/configure.ac

--- strace-4.5.15.orig/configure.ac	2007-07-14 19:25:25.000000000 +0200

+++ strace-4.5.15/configure.ac	2007-07-14 19:27:35.000000000 +0200

@@ -91,14 +91,14 @@

 	arch=hppa

 	AC_DEFINE([HPPA], 1, [Define for the HPPA architecture.])

 	;;

-sh)

-	arch=sh

-	AC_DEFINE([SH], 1, [Define for the SH architecture.])

-	;;

 sh64)

 	arch=sh64

 	AC_DEFINE([SH64], 1, [Define for the SH64 architecture.])

 	;;

+sh*)

+	arch=sh

+	AC_DEFINE([SH], 1, [Define for the SH architecture.])

+	;;

 x86?64*)

 	arch=x86_64

 	AC_DEFINE([X86_64], 1, [Define for the AMD x86-64 architecture.])