Fix the compilation instructions of phobos and compiler to use posix.mak on unix-like systems

This fixes the instructions for the compiler and phobos that makes reference to `osx.mak` which was replaced by `posix.mak`.

Which tool can use to compile the docs website locally and check other issues?

Once you guys tell me how to compile the website locally, I can add these instructions to a Readme.md file so others can contribute more quickly.