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 gcc-3.4.6.orig/gcc/config/mips/t-linux gcc-3.4.6/gcc/config/mips/t-linux

 --- gcc-3.4.6.orig/gcc/config/mips/t-linux	1970-01-01 01:00:00.000000000 +0100

 +++ gcc-3.4.6/gcc/config/mips/t-linux	2007-08-15 23:08:15.000000000 +0200

 @@ -0,0 +1,3 @@

 +# Compile crtbegin/end with xgot so it works for both

 +# normal and large GOTs.

 +CRTSTUFF_T_CFLAGS = -mxgot