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.])