- was PREFIX/site, now PROJECT/site
This avoids several issues when installing OpenFOAM in clusters
without an intermediate OpenFOAM-specific installation prefix.
The 'site' directory may have a reserved meaning in these situations
and it is undesirable to 'leak' upwards into the parent directory to
look for configuration files.
Placing the default within the project directory avoids this.
Alternative locations can be given via the WM_PROJECT_SITE variable.