Updated BUILDING.md

[ghost]

• Removed stray spaces

• In Linux section

  • Changed apt-get to apt
  • Since most distros ship GCC 8 by default now, no need to install GCC 7.
  • Added CMake installation
  • No need to specify generator
  • Use CMake directly to build instead of Make

[mikekazakov]

Thank you very much for the fixes!