build: dont define HAVE_QRENCODE at all in case not present

Before we got an error when libqrencode was not installed:
`src/ui/console.c:52:10: fatal error: qrencode.h: No such file or
directory`

Which looked like HAVE_QRENCODE was true.

config.status showed:
`config.status:D["HAVE_QRENCODE"]=" 0"`

Holger pointed to me that there is not just true and false but defined
and undefined.
So one solution was to use `#if HAVE_QRENCODE == 1` to check for the
actual value.

Or dont define HAVE_QRENCODE in the non present case at all.

In all the other HAVE_ variables we use this approach.
I think i at first chose the wrong way out of confusion with BUILD_ and
HAVE_.

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