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 glibc-2.10.1.orig/sysdeps/sh/sh4/fpu/fpu_control.h glibc-2.10.1/sysdeps/sh/sh4/fpu/fpu_control.h

 --- glibc-2.10.1.orig/sysdeps/sh/sh4/fpu/fpu_control.h	2001-07-06 06:56:04.000000000 +0200

 +++ glibc-2.10.1/sysdeps/sh/sh4/fpu/fpu_control.h	2009-11-13 00:51:26.000000000 +0100

 @@ -45,6 +45,7 @@

  #define _FPU_GETCW(cw) __asm__ ("sts fpscr,%0" : "=r" (cw))

  #if defined __GNUC__

 +extern void __set_fpscr(unsigned long);

  #define _FPU_SETCW(cw) __set_fpscr ((cw))

  #else

  #define _FPU_SETCW(cw) __asm__ ("lds %0,fpscr" : : "r" (cw))