Make configure hard code path to swig

I've set it up to just try "swig" if configure hasn't been run since
syncing to this change, so users who sync and have no swig issues won't
have to do anything new.

Fixes #1329.

configure

@@ -22,6 +22,16 @@ while true; do
   # Retry
 done
 
+## Find swig path
+if [ -z "$SWIG_PATH" ]; then
+  SWIG_PATH=`type -p swig 2> /dev/null`
+fi
+if [[ ! -e "$SWIG_PATH" ]]; then
+  echo "Can't find swig.  Ensure swig is in \$PATH or set \$SWIG_PATH."
+  exit 1
+fi
+echo "$SWIG_PATH" > tensorflow/tools/swig/swig_path
+
 # Invoke python_config and set up symlinks to python includes
 (./util/python/python_config.sh --setup "$PYTHON_BIN_PATH";) || exit -1
 

tensorflow/tools/swig/.gitignore

@@ -0,0 +1 @@
+swig_path

tensorflow/tools/swig/swig.sh

@@ -14,4 +14,12 @@
 # limitations under the License.
 # ==============================================================================
 
-swig "$@"
+# If possible, read swig path out of "swig_path" generated by configure
+SWIG=swig
+SWIG_PATH=tensorflow/tools/swig/swig_path
+if [ -e $SWIG_PATH ]; then
+  SWIG=`cat $SWIG_PATH`
+fi
+
+# If this line fails, rerun configure to set the path to swig correctly
+"$SWIG" "$@"