How to specify -stdlib when using clang++

[watiko]

It is possible to specify -stdlib by writing the following in poac.toml, but it will not work when CXX=g++.

[profile]
cxxflags = ["-stdlib=libc++"]

I felt it would be useful if there was a way to make it work for both CXX=clang++ and CXX=g++. I'm not sure what the best way is, but I wrote notes of what I came up with.

• Add CXXFLAGS environment variable (similar to RUSTFLAGS)
• Allow CXX to be fixed in poac.toml (profile.cxx?) to avoid confusion if only one cxx is assumed.

[ken-matsui]

@watiko For conditional CXXFLAGS, I think adding cfg works better. For example:

[target.'cfg(compiler = "clang++")'.profile]
cxxflags = ["-stdlib=libc++"]

For fixing CXX to a specific compiler, I am not sure that we should do it. Let me think of it.