_fromGlibCTest example

        $features_header_file = '/usr/include/features.h';
        if (!(@file_exists($features_header_file)
            && @is_readable($features_header_file))
        ) {
            return array();
        }
        if (!@file_exists('/usr/bin/cpp') || !@is_executable('/usr/bin/cpp')) {
            return $this->_parseFeaturesHeaderFile($features_header_file);
        } // no cpp
        return $this->_fromGlibCTest();
    }

    function _parseFeaturesHeaderFile($features_header_file)
    {
        $features_file = fopen($features_header_file, 'rb');
        while (!feof($features_file)) {
            $line = fgets($features_file, 8192);
            if (!$this->_IsADefinition($line)) {
                continue;
            }
            if (strpos($line, '__GLIBC__')) {