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>

 commit 25bb77587944768581225d1c0048facab32cf019

 Author: Roberto Bagnara <bagnara@cs.unipr.it>

 Date:   Wed May 18 09:35:36 2011 +0200

     Moved misplaced conditional.

 diff --git a/interfaces/Java/tests/Makefile.am b/interfaces/Java/tests/Makefile.am

 index 64ba864..607bbc6 100644

 --- a/interfaces/Java/tests/Makefile.am

 +++ b/interfaces/Java/tests/Makefile.am

 @@ -51,6 +51,8 @@ Parma_Polyhedra_Library_test2.java \

  PPL_Test.java \

  ppl_java_tests_common

 +if ENABLE_SHARED

 +

  check_JAVA = \

  ppl_java_generated_tests.java \

  Test_Executor.java \

 @@ -62,8 +64,6 @@ Parma_Polyhedra_Library_test1.java \

  Parma_Polyhedra_Library_test2.java \

  PPL_Test.java

 -if ENABLE_SHARED

 -

  CLASSPATH = ../ppl_java.jar:.

  JAVACFLAGS = -classpath $(CLASSPATH)