Make build.py work when ninja isn't in the path.
Make build.py work when ninja isn't in the path.

antiagainst/shaderc,dneto0/shaderc,dneto0/shaderc,AWoloszyn/shaderc,dneto0/shaderc,AWoloszyn/shaderc,AWoloszyn/shaderc,dneto0/shaderc,antiagainst/shaderc,antiagainst/shaderc,antiagainst/shaderc,fuchsia-mirror/third_party-shaderc,fuchsia-mirror/third_party-shaderc,fuchsia-mirror/third_party-shaderc