Add missing declarations for Google Tests flags
Add declarations for install_failure_signal_handler and flagfile.
Fix Clang warnings:
warning: no previous extern declaration for non-static variable | |
'FLAGS_gtest_install_failure_signal_handler' [-Wmissing-variable-declarations] | |
warning: no previous extern declaration for non-static variable | |
'FLAGS_gtest_flagfile' | [-Wmissing-variable-declarations] |