Fix the default assignment of DEBUG_LEVEL in Makefile
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

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