Add `Conditional` wrapper to gtest
This follows an initial proposal for an 'EqIff` matcher. `Conditional` was considered more precise as an EqIff() matcher may suffer from `Iff` not being universally understood.
PiperOrigin-RevId: 383407665
- appends "_" to internal macro names (by Markus Heule).
- makes Google Test work with newer versions of tools on Symbian and Windows CE (by Mika Raento).
- adds the (ASSERT|EXPECT)_NO_FATAL_FAILURE macros (by Markus Heule).
- changes EXPECT_(NON|)FATAL_FAILURE to catch failures in the current thread only (by Markus Heule).
- adds the EXPECT_(NON|)FATAL_FAILURE_ON_ALL_THREADS macros (by Markus Heule).
- adds GTEST_HAS_PTHREAD and GTEST_IS_THREADSAFE to indicate the availability of <pthread.h> and Google Test's thread-safety (by Zhanyong Wan).
- adds scons/SConscript for building with scons (by Joi Sigurdsson).
- adds src/gtest-all.cc for building Google Test from a single file (by Markus Heule).
- updates the xcode project to include new tests (by Preston Jackson).
