configure: define preprocessor macros for SCSI and MPATH

This fixes building these backends when explicitly enabled
on the command line.

Exposed by commit a659559b which started adding
--with-storage-mpath in the spec file.

https://bugzilla.redhat.com/show_bug.cgi?id=1346724

configure.ac

@@ -1851,7 +1851,7 @@ else
 fi
 AM_CONDITIONAL([WITH_STORAGE_ISCSI], [test "$with_storage_iscsi" = "yes"])
 
-if test "$with_storage_scsi" = "check"; then
+if test "$with_storage_scsi" = "check" || test "$with_storage_scsi" = "yes"; then
    with_storage_scsi=yes
 
    AC_DEFINE_UNQUOTED([WITH_STORAGE_SCSI], 1,
@@ -1859,7 +1859,7 @@ if test "$with_storage_scsi" = "check"; then
 fi
 AM_CONDITIONAL([WITH_STORAGE_SCSI], [test "$with_storage_scsi" = "yes"])
 
-if test "$with_storage_mpath" = "check"; then
+if test "$with_storage_mpath" = "check" || test "$with_storage_mpath" = "yes"; then
    if test "$with_linux" = "yes"; then
       with_storage_mpath=yes