Implemented the .INCLUDE_DIRS special variable. It expands to a list

of directories that make searches for included makefiles.

tests/scripts/variables/INCLUDE_DIRS

@@ -0,0 +1,46 @@
+#                                                                    -*-perl-*-
+$description = "Test the .INCLUDE_DIRS special variable.";
+
+$details = "";
+
+use Cwd;
+
+$dir = cwd;
+$dir =~ s,.*/([^/]+)$,../$1,;
+
+# Test #1: The content of .INCLUDE_DIRS depends on the platform for which
+#          make was built. What we know for sure is that it shouldn't be
+#          empty.
+#
+run_make_test('
+ifeq ($(.INCLUDE_DIRS),)
+$(warning .INCLUDE_DIRS is empty)
+endif
+
+.PHONY: all
+all:;@:
+',
+'',
+'');
+
+
+# Test #2: Make sure -I paths end up in .INCLUDE_DIRS.
+#
+run_make_test('
+ifeq ($(dir),)
+$(warning dir is empty)
+endif
+
+ifeq ($(filter $(dir),$(.INCLUDE_DIRS)),)
+$(warning .INCLUDE_DIRS does not contain $(dir))
+endif
+
+.PHONY: all
+all:;@:
+',
+"-I$dir dir=$dir",
+'');
+
+
+# This tells the test driver that the perl test script executed properly.
+1;