Do not compile scsi ioctl decoding if <scsi/sg.h> is not available

Add a generic test for <scsi/sg.h> availability to fix build with
Bionic libc that does not provide <scsi/sg.h>.

* configure.ac (AC_CHECK_HEADERS): Add scsi/sg.h.
* ioctl.c (ioctl_decode): Do not call scsi_ioctl if !HAVE_SCSI_SG_H.
* scsi.c: Do not compile scsi ioctl decoding if !HAVE_SCSI_SG_H.

Reported-by: Elliott Hughes <enh@google.com>

configure.ac

@@ -220,6 +220,7 @@ AC_CHECK_HEADERS(m4_normalize([
 	mqueue.h
 	netinet/sctp.h
 	poll.h
+	scsi/sg.h
 	stropts.h
 	sys/acl.h
 	sys/asynch.h

ioctl.c

@@ -86,8 +86,10 @@ ioctl_decode(struct tcb *tcp, long code, long arg)
 	case 0x03:
 	case 0x12:
 		return block_ioctl(tcp, code, arg);
+#ifdef HAVE_SCSI_SG_H
 	case 0x22:
 		return scsi_ioctl(tcp, code, arg);
+#endif
 	case 'L':
 		return loop_ioctl(tcp, code, arg);
 	case 'M':

scsi.c

@@ -27,8 +27,11 @@
  */
 
 #include "defs.h"
-#include <sys/ioctl.h>
-#include <scsi/sg.h>
+
+#ifdef HAVE_SCSI_SG_H
+
+# include <sys/ioctl.h>
+# include <scsi/sg.h>
 
 static const struct xlat sg_io_dxfer_direction[] = {
 	XLAT(SG_DXFER_NONE),
@@ -137,3 +140,5 @@ scsi_ioctl(struct tcb *tcp, long code, long arg)
 	}
 	return 1;
 }
+
+#endif /* HAVE_SCSI_SG_H */