accel: Allow to build QEMU without TCG or KVM support

Instead of deny build of QEMU without a default accelerator, simply
report an error when the user haven't passed -accel or -machine accel=
and TCG and KVM isn't builtin.

./configure already check that at least one accelerator is available.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/accel.c

@@ -91,7 +91,9 @@ void configure_accelerator(MachineState *ms, const char *progname)
 #elif defined(CONFIG_KVM)
             accel = "kvm";
 #else
-#error "No default accelerator available"
+            error_report("No accelerator selected and"
+                         " no default accelerator available");
+            exit(1);
 #endif
         }
     }