Make it harder for users to run debug builds in production

Summary:
I see a lot of users compiling RocksDB with `make` or `make all` and then using those binaries in production. They end up running debug builds :(

This diff makes it harder for them:
1. I added an explicit warning to INSTALL.md
2. When you compile with `make all`, your resulting library will be librocksdb_debug.a
3. I also print out a warning when you compile in debug mode.

Hopefully should be enough :)

Test Plan: none

Reviewers: rven, yhchiang, kradhakrishnan, anthony, sdong, dhruba, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D48093

INSTALL.md

@@ -1,19 +1,24 @@
 ## Compilation
 
+**Important**: If you plan to run RocksDB in production, don't compile using default 
+`make` or `make all`. That will compile RocksDB in debug mode, which is much slower
+than release mode.
+
 RocksDB's library should be able to compile without any dependency installed,
 although we recommend installing some compression libraries (see below).
 We do depend on newer gcc/clang with C++11 support.
 
 There are few options when compiling RocksDB:
 
-* [recommended] `make static_lib` will compile librocksdb.a, RocksDB static library.
+* [recommended] `make static_lib` will compile librocksdb.a, RocksDB static library. Compiles static library in release mode.
 
-* `make shared_lib` will compile librocksdb.so, RocksDB shared library.
+* `make shared_lib` will compile librocksdb.so, RocksDB shared library. Compiles shared library in release mode.
 
-* `make check` will compile and run all the unit tests
+* `make check` will compile and run all the unit tests. `make check` will compile RocksDB in debug mode.
 
 * `make all` will compile our static library, and all our tools and unit tests. Our tools
-depend on gflags. You will need to have gflags installed to run `make all`.
+depend on gflags. You will need to have gflags installed to run `make all`. This will compile RocksDB in debug mode. Don't
+use binaries compiled by `make all` in production.
 
 * By default the binary we produce is optimized for the platform you're compiling on
 (-march=native). If you want to build a portable binary, add 'PORTABLE=1' before

Makefile

@@ -76,6 +76,8 @@ endif
 ifeq ($(DEBUG_LEVEL),0)
 OPT += -DNDEBUG
 DISABLE_WARNING_AS_ERROR=1
+else
+$(warning Warning: Compiling in debug mode. Don't use the resulting binary in production)
 endif
 
 #-----------------------------------------------
@@ -322,10 +324,14 @@ TOOLS = \
 
 BENCHMARKS = db_bench table_reader_bench cache_bench memtablerep_bench
 
-# The library name is configurable since we are maintaining libraries of both
-# debug/release mode.
+# if user didn't config LIBNAME, set the default
 ifeq ($(LIBNAME),)
+# we should only run rocksdb in production with DEBUG_LEVEL 0
+ifeq ($(DEBUG_LEVEL),0)
         LIBNAME=librocksdb
+else
+        LIBNAME=librocksdb_debug
+endif
 endif
 LIBRARY = ${LIBNAME}.a