configure: allow --cpu to be specified

And don't fail if we don't detect, it's not critical
for configure.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/configure b/configure
index d7ca77c..10a9a4c 100755
--- a/configure
+++ b/configure
@@ -129,6 +129,8 @@
 for opt do
   optarg=`expr "x$opt" : 'x[^=]*=\(.*\)'`
   case "$opt" in
+  --cpu=*) cpu="$optarg"
+  ;;
   --cc=*) CC="$optarg"
   ;;
   --extra-cflags=*) CFLAGS="$CFLAGS $optarg"
@@ -146,6 +148,7 @@
 done
 
 if test "$show_help" = "yes" ; then
+  echo "--cpu=                 Specify target CPU if auto-detect fails"
   echo "--cc=                  Specify compiler to use"
   echo "--extra-cflags=        Specify extra CFLAGS to pass to compiler"
   echo "--build-32bit-win=     Specify yes for a 32-bit build on Windows"
@@ -276,8 +279,7 @@
     cpu="sparc"
   ;;
   *)
-    echo "Unknown CPU"
-    exit 1;
+  echo "Unknown CPU"
   ;;
 esac