build: workaround behaviour regression in gnu make 4.3

We need the "$(space)" variable to contain a single whitespace
character. We do this by assigning and then appending an empty
string to the variable. Variable appends get separated by a
single whitespace historically, but GNU make 4.3 introduced a
behaviour regression.

https://lists.gnu.org/archive/html/bug-make/2020-01/msg00057.html

[quote]
* WARNING: Backward-incompatibility!
Previously appending using '+=' to an empty variable would
result in a value starting with a space. Now the initial
space is only added if the variable already contains some
value. Similarly, appending an empty string does not
add a trailing space.
[/quote]

This patch tries a new trick to get a single whitespace by
getting make to expand two non-existant variables separated
by a space.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>