LIBNAME in Makefile is not really configurable

Summary:
In new third-party release tool, `LIBNAME=<customized_library> make`
will not really change the LIBNAME.

However it's very odd that the same approach works with old third-party
release tools. I checked previous rocksdb version and both librocksdb.a
and librocksdb_debug.a were correctly generated and copied to the
right place.

Test Plan:
`LIBNAME=hello make -j32` generates hello.a
`make -j32` generates librocksdb.a

Reviewers: igor, sdong, haobo, dhruba

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15555

Makefile

@@ -97,7 +97,9 @@ BENCHMARKS = db_bench_sqlite3 db_bench_tree_db table_reader_bench
 
 # The library name is configurable since we are maintaining libraries of both
 # debug/release mode.
-LIBNAME = librocksdb
+ifeq ($(LIBNAME),)
+        LIBNAME=librocksdb
+endif
 LIBRARY = ${LIBNAME}.a
 MEMENVLIBRARY = libmemenv.a