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>

Respect environment CPPFLAGS when we run ./configure so we can inject

random -D things without having to set CFLAGS/ASFLAGS

diff -durN glibc-2.13.orig/Makeconfig glibc-2.13/Makeconfig

--- glibc-2.13.orig/Makeconfig	2009-05-09 23:57:56.000000000 +0200

+++ glibc-2.13/Makeconfig	2009-11-13 00:49:41.000000000 +0100

@@ -697,6 +697,7 @@

 	   $(foreach lib,$(libof-$(basename $(@F))) \

 			 $(libof-$(<F)) $(libof-$(@F)),$(CPPFLAGS-$(lib))) \

 	   $(CPPFLAGS-$(<F)) $(CPPFLAGS-$(@F)) $(CPPFLAGS-$(basename $(@F)))

+CPPFLAGS += $(CPPFLAGS-config)

 override CFLAGS	= -std=gnu99 $(gnu89-inline-CFLAGS) \

 		  $(filter-out %frame-pointer,$(+cflags)) $(+gccwarn-c) \

 		  $(sysdep-CFLAGS) $(CFLAGS-$(suffix $@)) $(CFLAGS-$(<F)) \