1.在编译Qtpdf时，需要clone pdfium
john@john-virtual-machine:~/work/qtpdf$ git submodule update --init --recursive
正克隆到 '/home/john/work/qtpdf/src/3rdparty/pdfium'...
fatal: unable to access 'https://pdfium.googlesource.com/pdfium/': Could not resolve host: pdfium.googlesource.com
fatal: 无法克隆 'https://pdfium.googlesource.com/pdfium' 到子模组路径 '/home/john/work/qtpdf/src/3rdparty/pdfium'
克隆 'src/3rdparty/pdfium' 失败。按计划重试
正克隆到 '/home/john/work/qtpdf/src/3rdparty/pdfium'...
fatal: unable to access 'https://pdfium.googlesource.com/pdfium/': Could not resolve host: pdfium.googlesource.com
fatal: 无法克隆 'https://pdfium.googlesource.com/pdfium' 到子模组路径 '/home/john/work/qtpdf/src/3rdparty/pdfium'
第二次尝试克隆 'src/3rdparty/pdfium' 失败，退出
这种问题肯定是网络问题了，但是我这个是Ubuntu1804的虚拟机，调了半天的网络适配器还是不行（已架设好梯子）
2.curl测试https是否访问正常
curl https://www.baidu.com
发现测试正常。
3.网址也能正常访问
4.更换Windows的Git安装版工具clone
出现OpenSSL SSL_connect: SSL_ERROR_SYSCALL 错误
5.更换 PortableGit 后下载正常
Administrator@DESKTOP-QA09TT2 MINGW64 /work
$ git clone https://pdfium.googlesource.com/pdfium
Cloning into 'pdfium'...
remote: Sending approximately 117.54 MiB ...
remote: Total 162337 (delta 127442), reused 162337 (delta 127442)
Receiving objects: 100% (162337/162337), 117.54 MiB | 1.93 MiB/s, done.
Resolving deltas: 100% (127442/127442), done.
Updating files: 100% (3769/3769), done.