- foamConfigurePaths (1812) now supports a -gcc option.
  Eg,
      foamConfigurePaths -gcc gcc-4.8.5

  This avoids fragile complier versioning that was previously
  hardcoded into the legacyCompile script.

- suggest build with llvm, since this allows possible reuse with mesa

- This may occur with some frequency.

- Add some comments and reduce the number of variable at the top.