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>

 Original patch from gentoo: ../../dev/crosstool-NG/ct-ng.trunk/patches/strace/4.5.17/150-undef-CTL_PROC.patch

 -= BEGIN original header =-

 diff -dur strace-4.5.16.orig/system.c strace-4.5.16/system.c

 -= END original header =-

 diff -durN strace-4.5.17.orig/system.c strace-4.5.17/system.c

 --- strace-4.5.17.orig/system.c	2007-01-17 00:22:36.000000000 +0100

 +++ strace-4.5.17/system.c	2008-10-24 21:30:13.000000000 +0200

 @@ -1612,7 +1612,9 @@

  	{ CTL_KERN, "CTL_KERN" },

  	{ CTL_VM, "CTL_VM" },

  	{ CTL_NET, "CTL_NET" },

 +#ifdef CTL_PROC

  	{ CTL_PROC, "CTL_PROC" },

 +#endif

  	{ CTL_FS, "CTL_FS" },

  	{ CTL_DEBUG, "CTL_DEBUG" },

  	{ CTL_DEV, "CTL_DEV" },