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>

Index: ltrace-0.5.3/configure

===================================================================

--- ltrace-0.5.3/configure

+++ ltrace-0.5.3/configure

@@ -78,7 +78,10 @@ fi

 rm -f conftest.c a.out

 CFLAGS='-g -O2'

-LIBS='-lelf -lsupc++ -liberty '

+LIBS='-lelf'

+[ -n "$HAVE_LIBSUPC__" ] && LIBS="$LIBS -lsupc++"

+[ -n "$HAVE_LIBIBERTY" ] && LIBS="$LIBS -liberty"

+

 INSTALL='install -c'

 iquote='-iquote '

 iquoteend=''