Fix the default assignment of DEBUG_LEVEL in Makefile

Summary:
In the current make file, DEBUG_LEVEL is forced to set to 1
instead of default to 1.  This patch fix this issue.

Test Plan:
DEBUG_LEVEL=2 make db_bench -j32
DEBUG_LEVEL=0 make db_bench -j32

DEBUG_LEVEL=0 make release -j32
And see whether there's a warning pops up.

Reviewers: MarkCallaghan, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D48933

Makefile

@@ -33,7 +33,9 @@ quoted_perl_command = $(subst ','\'',$(perl_command))
 # with debug level 0. To compile with level 0, run `make shared_lib`,
 # `make install-shared`, `make static_lib`, `make install-static` or
 # `make install`
-DEBUG_LEVEL=1
+
+# Set the default DEBUG_LEVEL to 1
+DEBUG_LEVEL?=1
 
 ifeq ($(MAKECMDGOALS),dbg)
 	DEBUG_LEVEL=2