This change improves spirv-fuzz CMake code to be more compatible with other projects that might want to include spirv-fuzz as a sub-project.
* Add a CMake option for building spirv-fuzz.
* We now check if protobuf targets are already available.
* We no longer specify `-DGOOGLE_PROTOBUF_NO_RTTI -DGOOGLE_PROTOBUF_USE_UNALIGNED=0`; a newer version of protobuf does not require this. Note that we probably should have specified this for protobuf targets as well, but this is no longer needed.
* Updated protobuf version in Kokoro scripts and README.md.
Updated script to work with python3 and python2.
Added required tools.
We added a section to the readme to mention the tools that are needed to
build and test spirv-tools. For the compiler, the compilers used by the
bots are mentioned.
The bots have been changed. The windows bots will not use python 3.6 for testing. The other bots will still use python 2.7. Both Python2 and Python3 will be tested.
Fixes#2407.
Fixes#1856.
