Refactor build_tools/build_detect_version

[sepeth]

Fixes #55, and retains the fix for #11

• I didn't use awk, same goal can be achieved with cat, and it seems easier to read.
• Checking whether git exists is safer with "command -v"
• Last check about build_version.cc is changed/missing is effectively the same with overriding it. I directly write the output to util/build_version.cc

[liukai]

New version looks much more clear, thank you!