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>

do not bother running ldconfig on DESTDIR.  it wants to write the temp cache

file outside of the chroot.  doesnt matter anyways as we wont use the cache

results (portage will rebuild cache), so running ldconfig is simply a waste

of time.

diff -durN glibc-2.12.1.orig/Makefile glibc-2.12.1/Makefile

--- glibc-2.12.1.orig/Makefile	2009-04-23 20:23:03.000000000 +0200

+++ glibc-2.12.1/Makefile	2009-11-13 00:49:49.000000000 +0100

@@ -116,6 +116,7 @@

 	rm -f $(symbolic-link-list)

 install:

+dont-bother-with-destdir:

 	-test ! -x $(common-objpfx)elf/ldconfig || LC_ALL=C LANGUAGE=C \

 	  $(common-objpfx)elf/ldconfig $(addprefix -r ,$(install_root)) \

 				       $(slibdir) $(libdir)