[SV 59870] define/undefine prerequisites are not target-specific vars

* src/read.c (parse_var_assignment): If called in a target-specific
variable context don't allow define/undefine as variable assignments.
* test/scripts/variables/define: Add a test.
* test/scripts/variables/undefine: Add a test.

tests/scripts/variables/define

@@ -279,4 +279,22 @@ hello
 world
 ');
 
+# Ensure that define can be a target when not appearing in a variable
+# definition context.  See SV 59870
+
+run_make_test(q!
+define = define
+
+$(define) : ;@echo $@
+
+%:define
+
+all: define foo
+
+%.x : define
+
+foo:;
+!,
+    '', "define\n");
+
 1;