configure: Make only once with pseudo-"in source tree" builds

configure: Make only once with pseudo-"in source tree" builds

Pseudo-"in source tree" build used to run make in the build directory
as many times as goals. Worse, although .NOTPARALLEL is specified,
it does not work for patterns, and run make in parallel, which can break
things.

Add a new rule "build", and let it call make. The pattern rule only
needs to specify "build" as its prerequisite and have a no-op recipe so
that it does more than canceling built-in implicit rules.

Fixes: dedad02720 ("configure: add support for pseudo-"in source tree" builds")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-ID: <20231119101604.47325-1-akihiko.odaki@daynix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>