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-4.5.16.orig/process.c strace-4.5.16/process.c

 --- strace-4.5.16.orig/process.c	2007-01-11 23:08:38.000000000 +0100

 +++ strace-4.5.16/process.c	2007-07-14 19:19:58.000000000 +0200

 @@ -2685,7 +2685,6 @@

         { 4*REG_GBR,            "4*REG_GBR"                             },

         { 4*REG_MACH,           "4*REG_MACH"                            },

         { 4*REG_MACL,           "4*REG_MACL"                            },

 -       { 4*REG_SYSCALL,        "4*REG_SYSCALL"                         },

         { 4*REG_FPUL,           "4*REG_FPUL"                            },

         { 4*REG_FPREG0,         "4*REG_FPREG0"                          },

         { 4*(REG_FPREG0+1),     "4*REG_FPREG1"                          },