Bazel compile.sh: test for existence of the protoc compiler

Tested with removing the protoc binaries from a github copy.

Fixes #128.

--
MOS_MIGRATED_REVID=91297812
diff --git a/compile.sh b/compile.sh
index 1fe218b..c4ff9bf3 100755
--- a/compile.sh
+++ b/compile.sh
@@ -198,6 +198,9 @@
   done
 esac
 
+[[ -x "${PROTOC-}" ]] \
+    || fail "Protobuf compiler not found in ${PROTOC-}"
+
 mkdir -p fromhost
 if [ ! -f fromhost/BUILD ]; then
   write_fromhost_build