Better defaults for FMILibrary variables

If an environment variable named 'FMIDIR' exists, we use it to establish
an initial value for incdirs and libdirs.  The command line option
can override.

This is useful because if you want to install PyFMI via pip, it
doesn't quite work to use the 'install_options' flag because it
seems to apply it to all dependencies (although I will test this
again).