Summary:
Introduced option to dump malloc statistics using new option flag.
    Added new command line option to db_bench tool to enable this
    funtionality.
    Also extended build to support environments with/without jemalloc.

Test Plan:
1) Build rocksdb using `make` command. Launch the following command
    `./db_bench --benchmarks=fillrandom --dump_malloc_stats=true
    --num=10000000` end verified that jemalloc dump is present in LOG file.
    2) Build rocksdb using `DISABLE_JEMALLOC=1  make db_bench -j32` and ran
    the same db_bench tool and found the following message in LOG file:
    "Please compile with jemalloc to enable malloc dump".
    3) Also built rocksdb using `make` command on MacOS to verify behavior
    in non-FB environment.
    Also to debug build configuration change temporary changed
    AM_DEFAULT_VERBOSITY = 1 in Makefile to see compiler and build
    tools output. For case 1) -DROCKSDB_JEMALLOC was present in compiler
    command line. For both 2) and 3) this flag was not present.

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D57321