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>

http://sourceware.org/ml/libc-alpha/2002-10/msg00392.html

diff -durN glibc-2.10.1.orig/glibc-ports-2.10.1/sysdeps/mips/fpu_control.h glibc-2.10.1/glibc-ports-2.10.1/sysdeps/mips/fpu_control.h

--- glibc-2.10.1.orig/glibc-ports-2.10.1/sysdeps/mips/fpu_control.h	2009-05-16 10:36:20.000000000 +0200

+++ glibc-2.10.1/glibc-ports-2.10.1/sysdeps/mips/fpu_control.h	2009-11-13 00:51:31.000000000 +0100

@@ -86,7 +86,7 @@

 #define _FPU_RC_UP      0x2

 #define _FPU_RC_DOWN    0x3

-#define _FPU_RESERVED 0xfe3c0000  /* Reserved bits in cw */

+#define _FPU_RESERVED 0xfebc0000  /* Reserved bits in cw */

 /* The fdlibm code requires strict IEEE double precision arithmetic,

diff -durN glibc-2.10.1.orig/ports/sysdeps/mips/fpu_control.h glibc-2.10.1/ports/sysdeps/mips/fpu_control.h