STYLE: cleanup compiler settings (issue #176)

- export/setenv WM_COMPILER_TYPE as suggested by Mattijs.

- for overall consistency, don't carp about an unset WM_COMPILER_TYPE,
  since this would only be on the first instance (prior to the
  export/setenv) and would be confusing about why/when this message
  may occur.

- reduce clutter: only use (system|ThirdParty) for WM_COMPILER_TYPE.
  Drop the old 'OpenFOAM' setting for WM_COMPILER_TYPE, which was
  transitional in early 2011.

- make the error messages more meaningful