Add maint.mk fetch gmock and gtest.
maint: Import gtest and gmock libraries. Updates can be made by maint.mk.
maint: maint.mk fetch acx_pthread from protobuf project. Add acx-pthread.mp4. Add DGTEST_HAS_PTHREAD flag for gtest when has pthread.
maint: Build libgmockgtest as check_ltlibraries for tests with gtest and gmock.
maint:  Exclude gmock and gtest from cpplint check. Add cpplint from maint.mk
as a target in eclipse CDT.