improve support for clang

[kazuho]

As of now, we have a code that emulates the behavior of gcc -wrapper in case clang (or other compiler) is being used.

However the argument parsing logic of the emulation is horrible; it just strips -o target and assumes that the last argument is the name of the source file. It is evident that such approach will fail in certain cases, e.g. when -Wl, is used after specifying the source file, or, when more than one source files are used.

We need to implement a better option parser like the one found in ccache or dist-cc.