benchmark

Add BENCHMARK_BUILD_32_BITS option and add builders to test it (#360)

* Add BENCHMARK_BUILD_32_BITS option and add builders to test it

* Attempt to fix travis configuration

* Make add_required_cxx_compiler_flag cause an error when the flag isn't supported

* add gcc-multilib dependancy on travis

* attempt to fix travis.yml parsing error

* Require g++-multilib instead of gcc-multilib

* Add 32 bit release configurations

* Attempt to fix libc++ travis build w/ 32 bits

* Work around CMake configuration failure on Travis