Fix building RocksDB for iOS (#4687)

Summary:
This PR contains the following fixes:

1. Fixing Makefile to support non-default locations of developer tools

2. Fixing compile error using a patch from https://github.com/facebook/rocksdb/pull/4007
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4687

Differential Revision: D13287263

Pulled By: riversand963

fbshipit-source-id: 4525eb42ba7b6f82af5f9bfb8e52fa4024e27ccc

Makefile

@@ -1935,7 +1935,8 @@ commit_prereq: build_tools/rocksdb-lego-determinator \
 ifeq ($(PLATFORM), IOS)
 # For iOS, create universal object files to be used on both the simulator and
 # a device.
-PLATFORMSROOT=/Applications/Xcode.app/Contents/Developer/Platforms
+XCODEROOT=$(shell xcode-select -print-path)
+PLATFORMSROOT=$(XCODEROOT)/Platforms
 SIMULATORROOT=$(PLATFORMSROOT)/iPhoneSimulator.platform/Developer
 DEVICEROOT=$(PLATFORMSROOT)/iPhoneOS.platform/Developer
 IOSVERSION=$(shell defaults read $(PLATFORMSROOT)/iPhoneOS.platform/version CFBundleShortVersionString)

db/db_impl.cc

@@ -2923,6 +2923,8 @@ void DumpRocksDBBuildVersion(Logger* log) {
                    ROCKSDB_MINOR, ROCKSDB_PATCH);
   ROCKS_LOG_HEADER(log, "Git sha %s", rocksdb_build_git_sha);
   ROCKS_LOG_HEADER(log, "Compile date %s", rocksdb_build_compile_date);
+#else
+    (void)log;  // ignore "-Wunused-parameter"
 #endif
 }