Return platform-specific modelf90 shared library name.

def _libname(self):
        """Return platform-specific modelf90 shared library name."""
        prefix = 'lib'
        suffix = '.so'
        if platform.system() == 'Darwin':
            suffix = '.dylib'
        if platform.system() == 'Windows':
            prefix = ''
            suffix = '.dll'
        return prefix + self.engine + suffix