configure: add --ninja option

On Windows it is not possible to invoke a Python script as $NINJA.
If ninja is present use it directly, while if it is not we can
keep using ninjatool.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed