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Move pthread dependency to test CMakeLists.txt
Fix #54 by correcting reference to CXX flags for variants.
Fix #50 by using nullptr and adding stricter warning.
Turn the global mutex into instance variables

To fix #52
Merge pull request #56 from google/pthread_dependency

Move pthread dependency to test CMakeLists.txt
Merge pull request #57 from google/flag_variants

Fix #54 by correcting reference to CXX flags for variants.
Merge pull request #58 from google/zero_null

Fix #50 by using nullptr and adding stricter warning.
Merge pull request #59 from zjx20/instance_mutex

Turn the global mutex into instance variables
Merge branch 'master' of github.com:google/benchmark