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>

YEM-20070519:

Patch from Bernhard Fischer <rep.dot.nop@gmail.com> on the uClibc mailing

list ( http://www.uclibc.org/lists/uclibc/2008-January/018940.html ) above

the 400-bits_sysnum_h.patch.

Index: uClibc/Makefile.in

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

--- uClibc/Makefile.in	(revision 18651)

+++ uClibc/Makefile.in	(working copy)

@@ -114,7 +116,7 @@ install: install_runtime install_dev

 RUNTIME_PREFIX_LIB_FROM_DEVEL_PREFIX_LIB=$(shell $(top_srcdir)extra/scripts/relative_path.sh $(DEVEL_PREFIX)lib $(RUNTIME_PREFIX)lib)

 # Installs header files.

-install_headers:

+install_headers: headers

 	$(INSTALL) -d $(PREFIX)$(DEVEL_PREFIX)include

 	printf ".svn\n.cvsignore\nCVS\n" > tar_exclude ; \

 	$(TAR) -chf - -X tar_exclude include \