make: run shell with pipefail

Without pipefail, it is possible to miss failures if the recipes
include pipes.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -14,6 +14,8 @@ SRC_PATH=.
 # we have explicit rules for everything
 MAKEFLAGS += -rR
 
+SHELL = /usr/bin/env bash -o pipefail
+
 # Usage: $(call quiet-command,command and args,"NAME","args to print")
 # This will run "command and args", and either:
 #  if V=1 just print the whole command and args