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>

 This patch is courtesy of OpenEmbedded, by Khem Raj <raj.khem@gmail.com>

 http://cgit.openembedded.org/cgit.cgi/openembedded/commit/?id=b85b779688f0abc389e3c25d06b54de8b86385c1

 Index: ltrace-0.5.3/Makefile.in

 ===================================================================

 --- ltrace-0.5.3.orig/Makefile.in	2009-10-23 22:06:08.130304691 -0700

 +++ ltrace-0.5.3/Makefile.in	2009-10-23 22:06:53.160369614 -0700

 @@ -39,6 +39,8 @@ ltrace:		main.o libltrace.a

  libltrace.a: 	sysdeps/sysdep.o $(OBJ)

  		$(AR) rcv $@ $^

 +$(OBJ):		sysdeps/sysdep.o

 +

  sysdeps/sysdep.o: dummy

  		$(MAKE) -C sysdeps/$(OS)