Disable CompressLevelCompaction() if Zlib is not supported

Summary: CompressLevelCompaction() depends on Zlib. We should skip it when zlib is not present.

Test Plan: `make check` without zlib

Reviewers: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D40401

db/db_test.cc

@@ -13053,6 +13053,9 @@ TEST_F(DBTest, EmptyCompactedDB) {
 }
 
 TEST_F(DBTest, CompressLevelCompaction) {
+  if (!Zlib_Supported()) {
+    return;
+  }
   Options options = CurrentOptions();
   options.compaction_style = kCompactionStyleLevel;
   options.write_buffer_size = 100 << 10;  // 100KB