需要安装GNUTLS：https://amon.org/gnutls
通过源码包编译安装：
下载：https://mod.gnutls.org/downloads/
最高版本：mod_gnutls-0.8.2 （2017-01-08）编译过程出错，尚未克服。
实证版本：mod_gnutls-0.7.5 （2016-05-28）
参考：https://amon.org/mod_gnutls-0-8-2
wget https://mod.gnutls.org/downloads/mod_gnutls-0.7.5.tar.bz2 && tar -xjvf mod_gnutls-0.7.5.tar.bz2 && cd mod_gnutls-0.7.5 export LIBGNUTLS_CFLAGS="-I/usr/include/gnutls" LIBGNUTLS_LIBS="-L/usr/lib -lgnutls" ./configure --with-apxs=/usr/local/apache2/bin/apxs make && make install
配置信息：
Configuration summary for mod_gnutls: * mod_gnutls version: 0.7.5 * Apache Modules directory: /usr/local/apache2/modules * GnuTLS Library version: * CFLAGS for GnuTLS: -I/usr/include/gnutls * LDFLAGS for GnuTLS: -L/usr/lib -lgnutls * SRP Authentication: yes * MSVA Client Verification: no * Build documentation: no
输出：
... Libraries have been installed in: /usr/local/apache2/modules ...
编译安装完成。
重启Apache之后，查看phpinfo：
... Apache Environment Variable Value HTTPS on SSL_VERSION_LIBRARY GnuTLS/3.5.8 SSL_VERSION_INTERFACE mod_gnutls/0.7.5 SSL_PROTOCOL TLS1.2 ...
参考：https://openrepos.net/content/inte/taskwarrior
参考：https://httpd.apache.org/docs/current/mod/mod_authn_dbm#authdbmuserfile
参考：https://www.linuxunbound.com/2011/07/using-gnutls-with-apache-httpd-2-2/
参考：https://qnalist.com/questions/6428858/users-httpd-dso-load-failed-when-using-mod-authn-dbm-for-berkeley-db
参考：https://www.experts-exchange.com/questions/21140440/mod-ssl-Cannot-open-SSLSessionCache-DBM-file
参考：https://qnalist.com/questions/986000/users-httpd-cannot-open-sslsessioncache-dbm-file
参考：http://addls.com/1%e4%b8%aaipn%e4%b8%aa%e5%9f%9f%e5%90%8dn%e4%b8%aa%e8%af%81%e4%b9%a6
参考：https://holmesian.org/linode-vps-centos-anyconnect
参考：https://ma.ttias.be/day-google-chrome-disables-http2-nearly-everyone-may-31st-2016/
参考：http://gnupg.10057.n7.nabble.com/GnuTLS-OpenSSL-support-for-TLS1-1-1-2-td26933
参考：https://www.linuxunbound.com/2011/07/using-gnutls-with-apache-httpd-2-2/
参考：https://sourceforge.net/p/mod-gnutls/mailman/mod-gnutls-support/
参考：http://www.outoforder.cc/projects/apache/mod_gnutls/docs/
