[SV 45728] Detect changes in .VARIABLES more accurately.

For performance, we only recompute .VARIABLES when (a) it's expanded
and (b) when its value will change from a previous expansion.  To
determine (b) we were checking the number of entries in the hash
table which used to work until we started undefining entries: now if
you undefine and redefine the same number of entries in between
expanding .VARIABLES, it doesn't detect any change.  Instead, keep
an increasing change number.
* variables.c: Add variable_changenum.
(define_variable_in_set, merge_variable_sets): Increment
variable_changenum if adding a new variable to the global set.
(undefine_variable_in_set): Increment variable_changenum if
undefining a variable from the global set.
(lookup_special_var): Test variable_changenum not the hash table.
* tests/scripts/variables/special: Test undefining variables.

tests/scripts/variables/special

@@ -14,14 +14,22 @@ X2 := $(sort $(filter FOO BAR,$(.VARIABLES)))
 
 BAR := bar
 
-all:
-	@echo X1 = $(X1)
-	@echo X2 = $(X2)
-	@echo LAST = $(sort $(filter FOO BAR,$(.VARIABLES)))
+all: ; @echo X1 = $(X1); echo X2 = $(X2); echo LAST = $(sort $(filter FOO BAR,$(.VARIABLES)))
 ',
                '', "X1 =\nX2 = FOO\nLAST = BAR FOO\n");
 
+# SV 45728: Test that undefining a variable is reflected properly
 
+&run_make_test('
+FOO := foo
+BAR := bar
+$(info one: $(sort $(filter FOO BAR BAZ,$(.VARIABLES))))
+undefine BAR
+BAZ := baz
+$(info two: $(sort $(filter FOO BAR BAZ,$(.VARIABLES))))
+all:;@:
+',
+               '', "one: BAR FOO\ntwo: BAZ FOO\n");
 
 # $makefile2 = &get_tmpfile;
 # open(MAKEFILE, "> $makefile2");
@@ -31,9 +39,9 @@ all:
 # X1 := $(sort $(.TARGETS))
 
 # all: foo
-# 	@echo X1 = $(X1)
-# 	@echo X2 = $(X2)
-# 	@echo LAST = $(sort $(.TARGETS))
+#       @echo X1 = $(X1)
+#       @echo X2 = $(X2)
+#       @echo LAST = $(sort $(.TARGETS))
 
 # X2 := $(sort $(.TARGETS))
 
@@ -56,16 +64,16 @@ define foo
 : foo-one\
 foo-two
 : foo-three
-	: foo-four
+        : foo-four
 endef
 
 orig: ; : orig-one
-	: orig-two \
+        : orig-two \
 orig-three \
-	orig-four \
-		orig-five \\\\
-	: orig-six
-	$(foo)
+        orig-four \
+                orig-five \\\\
+        : orig-six
+        $(foo)
 
 .RECIPEPREFIX = >
 test: ; : test-one
@@ -78,19 +86,19 @@ test-three \
 
 .RECIPEPREFIX =
 reset: ; : reset-one
-	: reset-two \
+        : reset-two \
 reset-three \
-	reset-four \
-		reset-five \\\\
-	: reset-six
-	$(foo)
+        reset-four \
+                reset-five \\\\
+        : reset-six
+        $(foo)
 ',
                'orig test reset',
                ': orig-one
 : orig-two \
 orig-three \
 orig-four \
-	orig-five \\\\
+        orig-five \\\\
 : orig-six
 : foo-one foo-two
 : foo-three
@@ -99,7 +107,7 @@ orig-four \
 : test-two \
 test-three \
 test-four \
-	test-five \\\\
+        test-five \\\\
 : test-six
 : foo-one foo-two
 : foo-three
@@ -108,7 +116,7 @@ test-four \
 : reset-two \
 reset-three \
 reset-four \
-	reset-five \\\\
+        reset-five \\\\
 : reset-six
 : foo-one foo-two
 : foo-three