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 -durN ltrace-0.4.orig/configure ltrace-0.4/configure

--- ltrace-0.4.orig/configure	2008-10-23 23:19:38.000000000 +0200

+++ ltrace-0.4/configure	2008-10-23 23:19:06.000000000 +0200

@@ -1392,6 +1392,9 @@

 HOST_OS="$host_os"

+case "${HOST_OS}" in

+  linux-*) HOST_OS=linux-gnu;;

+esac

 ac_ext=c

diff -durN ltrace-0.4.orig/configure.ac ltrace-0.4/configure.ac

--- ltrace-0.4.orig/configure.ac	2008-10-23 23:19:43.000000000 +0200

+++ ltrace-0.4/configure.ac	2008-10-23 23:18:44.000000000 +0200

@@ -7,6 +7,9 @@

 dnl Check host system type

 AC_CANONICAL_HOST

 HOST_OS="$host_os"

+case "${HOST_OS}" in

+  linux-*) HOST_OS=linux-gnu;;

+esac

 AC_SUBST(HOST_OS)

 dnl Checks for programs.