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 -ur uClibc-0.9.29/libc/inet/resolv.c uClibc-0.9.29-patched/libc/inet/resolv.c

 --- uClibc-0.9.29/libc/inet/resolv.c	2007-04-23 12:01:05.000000000 -0500

 +++ uClibc-0.9.29-patched/libc/inet/resolv.c	2007-05-09 18:05:33.563404419 -0500

 @@ -1700,7 +1700,7 @@

  int gethostent_r(struct hostent *result_buf, char *buf, size_t buflen,

  	struct hostent **result, int *h_errnop)

  {

 -    int ret;

 +    int ret = HOST_NOT_FOUND;

      __UCLIBC_MUTEX_LOCK(mylock);

      if (__gethostent_fp == NULL) {