Fix the build on OpenBSD (#488)

* rocksdb fails without timer_delete() - only use it when it exists

storage/rocksdb/CMakeLists.txt

@@ -9,6 +9,11 @@ IF (NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/rocksdb/Makefile")
   SKIP_ROCKSDB_PLUGIN("Missing Makefile in rocksdb directory. Try \"git submodule update\".")
 ENDIF()
 
+CHECK_LIBRARY_EXISTS(rt timer_delete "" HAVE_TIMER_DELETE)
+IF (HAVE_TIMER_DELETE)
+  ADD_DEFINITIONS(-DHAVE_TIMER_DELETE)
+ENDIF(HAVE_TIMER_DELETE)
+
 CHECK_FUNCTION_EXISTS(sched_getcpu  HAVE_SCHED_GETCPU)
 IF(HAVE_SCHED_GETCPU)
   ADD_DEFINITIONS(-DHAVE_SCHED_GETCPU=1 -DROCKSDB_SCHED_GETCPU_PRESENT)

storage/rocksdb/rdb_io_watchdog.cc

@@ -22,7 +22,7 @@
 #include <vector>
 
 /* Rdb_io_watchdog doesn't work on Windows [yet] */
-#if !defined(_WIN32) && !defined(__APPLE__)
+#ifdef HAVE_TIMER_DELETE
 
 namespace myrocks {
 

storage/rocksdb/rdb_io_watchdog.h

@@ -35,7 +35,7 @@
 namespace myrocks {
 
 // Rdb_io_watchdog does not support Windows ATM.
-#if !defined(_WIN32) && !defined(__APPLE__)
+#ifdef HAVE_TIMER_DELETE
 
 class Rdb_io_watchdog {
   const int RDB_IO_WRITE_BUFFER_SIZE = 4096;