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>

 Correctly check for g++ existence.

 Copyright 2007 Yann E. MORIN <yann.morin.1998@anciens.enib.fr>

 Licensed to you as dmalloc-5.5.2 is.

 diff -dur dmalloc-5.5.2.orig/configure dmalloc-5.5.2/configure

 --- dmalloc-5.5.2.orig/configure	2007-05-18 11:40:31.000000000 +0200

 +++ dmalloc-5.5.2/configure	2007-05-18 11:42:02.000000000 +0200

 @@ -2643,7 +2643,7 @@

  # see if we actually have a CXX program

 -if test "$ac_cv_prog_CXX" = "" -o ! -x "$ac_cv_prog_CXX"; then

 +if test "$ac_cv_prog_CXX" = "" -o ! -x `which "$ac_cv_prog_CXX"`; then

      { echo "$as_me:$LINENO: WARNING: could not find C++ compiler $ac_cv_prog_CXX" >&5

  echo "$as_me: WARNING: could not find C++ compiler $ac_cv_prog_CXX" >&2;}

      enable_cxx=no