support macos

[pinventado]

So far, here are steps we might need to do to support Mac

1. Install XQuartz https://www.xquartz.org/
2. Also install linux packages
   1. brew install coreutils
   2. brew install llvm (takes forever)
      1. ln -s "$(brew --prefix llvm)/bin/clang-format" "/usr/local/bin/clang-format"
      2. ln -s "$(brew --prefix llvm)/bin/clang-tidy" "/usr/local/bin/clang-tidy"
3. Install googletest
   git clone https://github.com/google/googletest
   cd googletest
   mkdir build
   cd build
   cmake .. -DCMAKE_CXX_STANDARD=17
   make
   make install
4. Change makefiles. Can we add/remove flags depending on the OS? Add these for Mac but not for Linux?
   1. Compilation: add -lm -I/opt/X11/include -lpthread -lX11 -lstdc++ -I/usr/X11R6/include -L/usr/X11R6/lib
   2. unittest: add -lm -lpthread -lX11 -I/usr/X11R6/include -L/usr/X11R6/lib