Enable no-fbcode RocksDB build

Summary: I want to use open source build rather than fbcode one. This enables me to run `ROCKSDB_NO_FBCODE=1 make` and run it with my system g++.

Test Plan:
ROCKSDB_NO_FBCODE=1 make
make

Reviewers: sdong, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23613

build_tools/build_detect_platform

@@ -46,7 +46,7 @@ PLATFORM_CXXFLAGS="-std=c++11"
 COMMON_FLAGS="-DROCKSDB_PLATFORM_POSIX"
 
 # Default to fbcode gcc on internal fb machines
-if [ -d /mnt/gvfs/third-party -a -z "$CXX" ]; then
+if [ -z "$ROCKSDB_NO_FBCODE" -a -d /mnt/gvfs/third-party ]; then
     FBCODE_BUILD="true"
     if [ -z "$USE_CLANG" ]; then
         CENTOS_VERSION=`rpm -q --qf "%{VERSION}" \