build: make sure the HAVE_ key is undefined on failure

Some particular checks can define the HAVE_ key on their own. To make sure
they work correctly when composed (with compose_checks) we force the HAVE_
key to be undefined if a check fails.

waftools/dependencies.py

@@ -95,6 +95,7 @@ the autodetection check failed.".format(self.identifier)
             self.success(self.identifier)
         else:
             self.fail()
+            self.ctx.undefine(inflector.define_key(self.identifier))
             self.fatal_if_needed()
 
     def enabled_option(self, identifier=None):