Add support for clang compiler

[akoshakji]

Currently the library does not compile with clang.

[biancagi]

The following lines are necessary to point clang to the gcc standard library. Remember to include these arguments in file .vscode/tasks.json:

"args": [
    "-stdlib=libc++",
    "-nostdinc++",
    "-I/opt/local/include/gcc/c++", 
    "-I/opt/local/include/gcc/c++/x86_64-apple-darwin20",
    "/opt/local/lib/libstdc++.dylib"
]