Meson: Advertise HAVE_OUTB

Rename __FLASHPROG_HAVE_OUTB__ to the more canonical HAVE_OUTB
and make sure Meson sets it on x86.

Change-Id: I67106292ea19ff49d9afe7d23b29e75c8e1ae0fc
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/495
diff --git a/pcidev.c b/pcidev.c
index e2dceb4..57bd20b 100644
--- a/pcidev.c
+++ b/pcidev.c
@@ -121,7 +121,7 @@
 		break;
 	case TYPE_IOBAR:
 		msg_pdbg("I/O\n");
-#if __FLASHPROG_HAVE_OUTB__
+#if HAVE_OUTB
 		if (!(supported_cycles & PCI_COMMAND_IO)) {
 			msg_perr("I/O BAR access requested, but device has I/O space accesses disabled.\n");
 			/* TODO: Abort here? */