function linkTextEnvVar( desc ) {
	if ( desc === '' ) {
		return '';
	}
	return 'DESC_LINK_TEXT=\'' + desc + '\'';
}
Returns a `DESC_LINK_TEXT` environment variable string.

@private
@param {string} desc - short description
@returns {string} environment variable string

@example
var s = linkTextEnvVar( 'cube root' );
// returns 'DESC_LINK_TEXT=\'cube root\''

@example
var s = linkTextEnvVar( '' );
// returns ''